Carbon-fiber-precursor fiber bundle, carbon fiber bundle, and uses thereof

ABSTRACT

A carbon fiber bundle, wherein an average single-fiber fineness is from 1.0 to 2.4 dtex and a roundness is from 0.7 to 0.9 in a shape of a cross-section perpendicular to a fiber axis of a single fiber; the roundness being determined with equation (1): roundness=4πS/L 2 , where S is a cross-sectional area of the single fiber and L is a circumferential length of the single fiber, and S and L are obtained by observing, under an SEM, the cross-section of the single fiber perpendicular to the fiber axis of the single fiber and analyzing the obtained image.

TECHNICAL FIELD

The present invention relates to a carbon-fiber-precursor fiber bundle,a method for flame proof treatment, a carbon fiber bundle and a methodof producing the carbon fiber bundle. The present invention also relatesto a carbon fiber prepreg, and particularly relates to a carbon fiberprepreg having a handling property and intensity reappearance suitablefor moldings in large size. Further, the present invention relates to amethod of molding a fiber reinforced fabric and a fiber reinforcedplastic.

BACKGROUND ART

When it is attempted to improve productivity by increasing a totalfineness of a carbon fiber bundle for the purpose of reducing productioncost of a carbon fiber, there are many problems in terms of practicaluse and production technology and the cost was not reduced sufficiently.

In order to solve these problems, Patent Document 1 has proposed thetechnology in which a scorch upon a flame proof treatment is reducedusing a carbon-fiber-precursor fiber bundle having a high roundness andfurther a large single-fiber fineness and a carbon fiber bundle whichcontains few interlaced single fibers, has excellent spreadability andproductivity despite of large total fineness is obtained.

Patent Document 2 has proposed a polymer which does not require theflame-proof treatment. Further, Patent Documents 3, 4 and 9 haveproposed the technology to enhance oxygen permeability of thecarbon-fiber precursor fiber to control an oxygen concentration in aflame-proof fiber evenly and enhance tensile strength and tensileelastic modulus of obtained carbon fiber by using a monomer having abulky side chain as a copolymerizable component of a copolymer.

Further, Patent Document 5 has proposed the technology to reduce thermalstorage inside the fiber bundle by progressing the flame-proof whileheated air is penetrated inside the fiber bundle on a mesh-shaped rollerfor PAN-based carbon-fiber-precursor fiber bundles.

Patent Document 6 has proposed the technology in which by measuring anisothermal exothermic curve of the carbon-fiber-precursor fiber bundleusing a heat flow type differential scanning calorimeter, a content of acarboxylic group-containing vinyl monomer is optimized, a cross-sectiondouble structure after the flame-proof treatment is reduced even whenburning at high speed is performed, and the productivity and the elasticmodulus of the carbon fiber bundle can be balanced. Patent Document 7has proposed the technology to produce the carbon fiber bundle with highperformance by copolymerizing acrylamide to obtain a highly hydrophilicpolyacrylonitrile copolymer.

Stabilization of the fiber in each step is also very important forreducing the production cost of the carbon fiber. For example, gelationof a spinning neat solution in a spinning step sometimes leads to a steptrouble, and it is required to enhance thermal stability of the spinningneat solution. In Patent Document 8, the thermal stability when thespinning neat solution is kept at high temperature of about 80° C. isexponentially enhanced by esterifying methacrylic acid that is anaccelerating component of the flame-proof reaction of the polymer.

A technique using a prepreg obtained by impregnating a fiber forreinforcement with a matrix resin composed mainly of a thermosettingresin is available as one of methods of molding fiber-reinforcedcomposite materials, and such a composite material is used for a widerange of uses from sport/leisure-related uses to uses for aircrafts. Thefiber-reinforced composite material using an intermediate base materialcomposed of the prepreg is formed by laminating the prepreg followed byheating or heating/pressurizing to cure the thermosetting resin that isthe matrix resin.

The technique using the prepreg is more excellent in fiber strengthutilization than VARTM method. When the molding in large size is formed,generally it is desirable that the matrix resin be high flow. The matrixresin with low flow causes an occurrence of voids. However, when thematrix resin is high flow, micro ondulation of the fiber occurs andmechanical physical property of the molding in large size is reduced.The mechanical physical property in the molding in large size largelydepends on its thickness, and when the thickness of the molding isincreased, compression strength is reduced. Patent Documents 10 and 11have proposed to prevent the reduction of various physical properties bymaking the matrix resin low flow.

When the fiber-reinforced fabric is used as a fiber base material, aresin film in which a resin was applied onto a film is attached to thefiber-reinforced fabric to make a prepreg, which is laminated in severallayers, and the layers are heated and pressurized in an autoclaveformation. In that case, the entire fabric is impregnated sufficientlywith the resin and a good molding is obtained. The impregnation with theresin is also very good regardless of structure and fibercross-sectional shape of the fiber-reinforced fabric. However, in RTMmolding and vacuum bag formation, the resin is injected into the fiberbase material, and thus, the resin having good fluidity and so-called alow viscosity resin is generally used as the resin. Thus, comparing withthe conventional autoclave formation, the cost of forming the fiber basematerial having a larger weight per unit area is described to beadvantageous, but the impregnation with the resin is largely influencedby the viscosity of the resin, the weight per unit area of the fiberfabric, inter-fiber voids, a single-fiber diameter, and the like, whichwas problematic.

Patent Document 12 has proposed the carbon fiber bundle having a ratioof a major axis and a minor axis (major axis/minor axis) of 1.05 to 1.6in a fiber cross-section of a single-fiber in the carbon fiber bundlescomposed of a single-fiber of multiple carbon fibers as the carbon fiberbundle that simultaneously satisfies bundle integrity, the impregnationwith the resin and cloth quality of obtained cloth and has the highstrength.

Patent Document 1: Japanese Unexamined Patent Application, PublicationNo. 2008-202207

Patent Document 2: Japanese Unexamined Patent Application, PublicationNo. H01-132832

Patent Document 3: Japanese Unexamined Patent Application, PublicationNo. H02-84505

Patent Document 4: Japanese Unexamined Patent Application, PublicationNo. 2006-257580

Patent Document 5: Japanese Unexamined Patent Application, PublicationNo. H02-6625

Patent Document 6: Japanese Unexamined Patent Application, PublicationNo. 2000-119341

Patent Document 7: Japanese Unexamined Patent Application, PublicationNo. H04-281008

Patent Document 8: Japanese Unexamined Patent Application, PublicationNo. 2007-204880

Patent Document 9: Japanese Unexamined Patent Application, PublicationNo. H02-84505

Patent Document 10: Japanese Unexamined Patent Application, PublicationNo. H01-161040

Patent Document 11: Japanese Unexamined Patent Application, PublicationNo. H02-169658

Patent Document 12: Japanese Unexamined Patent Application, PublicationNo. 2002-242027

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, the following drawbacks are found in the inventions describedin the above Patent Documents. In the technology in Patent Document 1,although the flame-proof step itself is shortened, the step ofperforming the flame-proof treatment to the polymer is required. Thus,the entire process of producing the carbon fiber is not shortened. Thestrength of the carbon fiber in Patent Document 2 is much lower thanthat of those using PAN and pitch as the raw material, and is notcapable of responding to the request from the market.

In the technology of Patent Documents 3, 4 and 10, although the oxygenpermeability inside the fiber is improved, shortening of the flame-proofstep did not lead to cost saving. When the copolymerizable component isa methacrylate ester-based monomer having a bulky alkyl group, there wasthe problem in that the precursor fiber bundle could not keepcompactness and homogeneity enough to assure the performancereappearance of the carbon fiber.

In the technology of Patent Document 5, there were the problems in thatit became difficult to penetrate the heated air in the thickcarbon-fiber-precursor fiber bundle and in that when a dischargepressure of the heated air was increased, interlace occurred inside thefiber bundle and spreadability of the fiber bundle when the prepreg isproduced was reduced.

In the technology of Patent Documents 6 and 7, the cross-section doublestructure of the flame-proof fiber can be reduced even at high speedburning in the carbon-fiber-precursor fiber bundle having a smallsingle-fiber fineness of about 1.2 dtex. However, the cross-sectiondouble structure cannot be reduced sometimes in thecarbon-fiber-precursor fiber bundle having the large single-fiberfineness of about 2.5 dtex.

In the technology of Patent Document 8, although the thermal stabilityof the spinning neat solution was exponentially enhanced, thecross-section double structure tended to be accelerated when theflame-proof treatment was performed in a period in which theproductivity of the carbon-fiber-precursor fiber bundle having the largesingle-fiber fineness was not impaired.

In the technology of Patents 10 and 11, by making the matrix resin lowflow, it is possible to prevent the occurrence of micro ondulation ofthe fiber and the reduction of the mechanical physical property due toit, but when this technology is applied to the formation of the moldingin large size, there is the problem in that the defects such as voidsoccur.

Patent Document 12 has provided the carbon fiber bundle excellent inimpregnation with the resin by the carbon fiber bundle of small numberof threads that are 3,000, but the total fineness of the carbon fiberbundle is small and thus it is very difficult to save the cost.

It is an object of the present invention to provide a carbon fiberbundle which has a large value of single-fiber fineness and excellentproductivity and which, despite this, contains few interlaced singlefibers therein and has excellent spreadability, and a carbon-fiberprecursor fiber suitable for the production thereof.

It is another object of the present invention to provide acarbon-fiber-precursor acryl fiber bundle that can efficiently produce ahigh quality carbon fiber bundle in which a cross-section doublestructure of a flame-proofed fiber is reduced in high speed burning evenwhen a single-fiber fineness is large, and a method of producing aflame-proofed fiber bundle using the precursor acryl fiber bundle.

It is another object of the present invention to provide acarbon-fiber-precursor acryl fiber bundle capable of providing a highquality carbon fiber bundle by performing an economical flame-proof heattreatment even when a single-fiber fineness is large, and aflame-proofed fiber using the precursor acryl fiber bundle, and a methodof producing a carbon fiber bundle using the precursor acryl fiberbundle.

It is another object of the present invention to provide a carbon fiberprepreg in which reduction of compression strength is small even when athickness of the molding after its formation is increased while highflow of a matrix resin is kept.

It is another object of the present invention to provide a carbon fiberbundle, a fiber-reinforced fabric and a method of forming afiber-reinforced plastic, which have high strand tensile strength andimpregnation even when a diameter of a single fiber is large.

Means for Solving the Problems

The above problems are solved by the present invention composed of thefollowing technical procedures [1] to [35].

[1] A carbon-fiber-precursor acryl fiber bundle composed of apolyacrylonitrile-based copolymer containing 95 to 99 mol % of anacrylonitrile unit and 1 to 5 mol % of a hydroxyalkyl (meth)acrylateunit, having a single-fiber fineness of 1.5 dtex or more and 5.0 dtex orless and having a roundness of 0.9 or less in a cross-section shapeperpendicular to a fiber axis of the single fiber:

wherein, the roundness is a value determined using a following equation(1), where S and L are a cross-sectional area and a circumferentiallength, respectively, of the single fiber, which are obtained byobserving, under an SEM, the cross-section of the single fiberperpendicular to the fiber axis of the single fiber and analyzing theobtained image:

Roundness=4πS/L ²   (1)

[2] The carbon-fiber-precursor acryl fiber bundle according to [1]above, wherein the single-fiber fineness is 1.5 dtex or more and 3.0dtex or less.

[3] The carbon-fiber-precursor acryl fiber bundle according to [1] or[2] above, wherein the roundness is 0.7 or more in the cross-sectionalshape perpendicular to the fiber axis of the single fiber.

[4] The carbon-fiber-precursor acryl fiber bundle according to [1] or[2] above, wherein a melting point under heat and humidity of thepolyacrylonitrile-based copolymer is 160 to 175° C.

[5] A carbon-fiber-precursor acryl fiber bundle, wherein a constantvelocity temperature rising exothermic curve at 30° C. or above and 450°C. or below measured at a temperature rising rate of 10° C./minute inair flow at 100 ml/minute at 30° C. and 0.10 MPa using a heat flux typedifferential scanning calorimeter satisfies the following condition:

a heat quantity Ja obtained by integrating an exothermic velocity at230° C. or above and 260° C. or below of the constant velocitytemperature rising exothermic curve is 100 kJ/kg or more and 250 kJ/kgor less; and heat quantity Jb obtained by integrating an exothermicvelocity at 260° C. or above and 290° C. or below is 550 kJ/kg or moreand 1050 kJ/kg or less.

[6] The carbon-fiber-precursor acryl fiber bundle according to [5]above, composed of a polyacrylonitrile-based copolymer composed of 95.0mol % or more and 99.0 mol % or less of an acrylonitrile unit and 1.0mol % or more and 5.0 mol % or less of a hydroxyalkyl (meth)acrylateunit.

[7] The carbon-fiber-precursor acryl fiber bundle according to [6]above, wherein a single-fiber fineness is 1.5 dtex or more and 5.0 dtexor less.

[8] The carbon-fiber-precursor acryl fiber bundle according to [6]above, wherein a water contact angle is 40° or more and 70° or less.

[9] The carbon-fiber-precursor acryl fiber bundle according to any of[5] to [8] above, wherein the heat quantity Ja is 160 kJ/kg or less.

[10] The carbon-fiber-precursor acryl fiber bundle according to any of[6] to [8] above, wherein an oxidation depth De obtained by a followingmethod is 4.0 μm or more and 6.0 μm or less in thepolyacrylonitrile-based copolymer:

1) the polyacrylonitrile-based copolymer is dissolved at a concentrationof 25% by mass in dimethylformamide to prepare a copolymer solution;

2) the copolymer solution is applied onto a glass plate;

3) the glass plate on which the copolymer solution was applied is driedin air at 120° C. for 6 hours to evaporate dimethylformamide and make afilm having a constant thickness in the range of 20 μm or more and 40 μmor less;

4) a flame-proof treatment is performed by treating the obtained filmwith heat in air at 240° C. for 60 minutes and further in air at 250° C.for 60 minutes to obtain a flame-proofed film;

5) the obtained flame-proofed film is embedded in a resin followed bybeing polished, and a cross-section perpendicular to a surface of thepolished flame-proofed film is observed at a magnification of 1500 usinga fluorescence microscope; and

6) An oxidation progressing part is observed as a relatively dark layerand an oxidation non-progressing part is observed as a relatively lightlayer in the cross-section, thus a distance from the surface of thepolished flame-proofed film to a boundary between the dark layer and thelight layer is measured at least at 5 points on one cross-section. Thesame measurement is further repeated on three cross-sections. Theirarithmetic average is used as the oxidation depth De (μm).

[11] A carbon-fiber-precursor acryl fiber bundle satisfying thefollowing conditions:

1) the single-fiber fineness is 2.0 dtex or more and 5.0 dtex or less;

2) a calorific value per unit mass at 215 to 300° C. obtained by themeasurement using a heat flux type differential scanning calorimeter is3200 kJ/kg or more (wherein, a temperature rising rate in themeasurement using the heat flux type differential scanning calorimeteris 2° C./minute and an atmosphere is air); and

3) a half-value width of solid ¹H-NMR spectra (measurement temperature160° C.) is 10.0 kHz or more and 14.5 kHz or less.

[12] The carbon-fiber-precursor acryl fiber bundle according to [11]above, composed of a polyacrylonitrile-based copolymer composed of 95.0mol % or more and 99.0 mol % or less of an acrylonitrile unit and 1.0mol % or more and 5.0 mol % or less of a hydroxyalkyl (meth)acrylateunit.

[13] The carbon-fiber-precursor acryl fiber bundle according to [12]above, wherein a calorific value at 215 to 300° C. obtained by themeasurement using a heat flux type differential scanning calorimeter is3300 kJ/kg or more.

[14] The carbon-fiber-precursor acryl fiber bundle according to [12]above, wherein a half-value width of solid ¹H-NMR spectra (measurementtemperature 160° C.) is 10.0 kHz or more and 13.5 kHz or less.

[15] A method for flame-proof treatment, wherein the flame-prooftreatment is performed to the carbon-fiber-precursor acryl fiber bundleaccording to any of [1], [2], [5] to [8] and [11] to [14] above under anoxidation atmosphere at temperature of 220° C. or above and 300° C. orbelow for 30 minutes or more and 90 minutes or less to obtain aflame-proofed fiber bundle having a fiber density of 1.35 g/cm³ or moreand 1.43 g/cm³ or less.

[16] A method of producing a carbon fiber bundle wherein a diameter Diis 8 μm or more and a roundness of a shape is 0.90 or less in across-section perpendicular to a fiber axis of a single fiber, wherein aflame-proof treatment is performed to the carbon-fiber-precursor acrylfiber bundle according to any of [1], [2], [5] to [8] and [11] to [14]above under an oxidation atmosphere at temperature of 220° C. or aboveand 300° C. or below for 30 minutes or more and 90 minutes or less toobtain a flame-proofed fiber bundle having a fiber density of 1.35 g/cm³or more and 1.43 g/cm³ or less, and wherein the flame-proofed fiberbundle is further carbonized at temperature of 800° C. or above and2000° C. or below in an inert gas:

wherein, the diameter Di is obtained by the following method:

1) preparation of sample,

wherein a carbon fiber bundle cut into a length of 5 cm is embedded inan epoxy resin (Epomount base: Epomount curing agent=100:9 (massratio)), and cut into a length of 2 cm to expose a cross-sectionalsurface, to which a mirror surface treatment is performed;

2) etching treatment of surface to be observed,

wherein further in order to clarify a contour of the fiber, an etchingtreatment is performed to the cross-sectional surface of the sample bythe following method:

Apparatus used: Plasma Etching Apparatus JP-170 manufactured by JEOLLtd.;

treatment condition: (Atmosphere gas: Ar/O₂=75/25, plasma output power:50 W, vacuum degree: about 120 Pa, treatment time period: 5 minutes);

3) observation under SEM,

wherein the cross-sectional surface of the samples obtained by 1) and 2)above is observed using SEM (PHILIPS FEI-XL20), and five photographs of5 or more fiber cross-sections on an image are taken randomly; and

4) Measurement of diameter of single fiber cross-section in carbon fiberbundle,

wherein for each sample, 20 single fiber cross-sections from the 5 SEMphotographs, wherein 3 or more single fiber cross-sections from onephotograph, are randomly selected, the contour of the fibercross-section is traced using image analysis software (product name:Image-ProPLUS manufactured by Nippon Roper K.K.), a major axis (maximumferet diameter) d of the cross-section is measured, and a mean value ofthe major axes of all single fiber cross-sections selected is used asthe diameter Di of the single fiber in the carbon fiber bundle.

[17] A carbon fiber bundle produced by the method according to [16]above, wherein an average single-fiber fineness is 1.0 to 2.4 dtex and aroundness is 0.7 or more and 0.9 or less in a shape of a cross-sectionperpendicular to a fiber axis of a single fiber.

[18] The carbon fiber bundle according to [17] above, wherein a diameterDi of the cross-section perpendicular to the fiber axis of the singlefiber is 8 to 20 μm.

[19] The carbon fiber bundle according to [17] above, having a pluralityof groove-shaped concavo-convex structures extending in a lengthwisedirection of the single fiber on the surface of the single fiber and adifference in height between a highest part and a lowest part in a rangeof a circumference length of 2 μm of the single fiber is 80 nm or less.

[20] The carbon fiber bundle according to any of [17] to [19] above,wherein a strand tensile strength is 4000 MPa or more.

[21] The carbon fiber bundle according to any of [17] to [19] above,wherein a strand tensile elastic modulus is 200 GPa or more.

[22] The carbon fiber bundle according to any of [17] to [19] above,wherein a total fineness is 30000 to 90000 dtex.

[23] A carbon fiber prepreg composed of a matrix resin and a carbonfiber bundle having a single-fiber fineness of 1.2 to 2.4 dtex and aroundness of 0.7 or more and 0.9 or less in a cross-sectionperpendicular to a fiber axis of a single fiber.

[24] The carbon fiber prepreg according to [23] above, wherein thecarbon fiber bundle is a PAN-based carbon fiber bundle.

[25] The carbon fiber prepreg according to [23] or [24] above, wherein adiameter Di perpendicular to the fiber axis of the single fiber in thecarbon fiber bundle is 8 to 20 μm.

[26] The carbon fiber prepreg according to any of [23] to [25] above,wherein a flow index of the matrix resin is 5000 Pa⁻¹ or more.

[27] The carbon fiber prepreg according to any of [23] to [26] above,wherein the matrix resin is an epoxy resin.

[28] The carbon fiber prepreg according to [27] above, wherein the epoxyresin comprises an epoxy resin having an oxazolidone ring structure.

[29] The carbon fiber prepreg according to [27] or [28] above, whereinthe epoxy resin comprises a thermoplastic resin.

[30] The carbon fiber prepreg according to any of [27] to [29] above,wherein the epoxy resin comprises dicyandiamide as a curing agent.

[31] The carbon fiber prepreg according to any of [27] or [30] above,wherein the epoxy resin comprises a urea compound as a curing aid.

[32] A unidirectional fiber-reinforced fabric, wherein the carbon fiberbundle according to any of [17] to [19] above is arranged in alongitudinal direction.

[33] The fiber-reinforced fabric according to [32] above, which is aunidirectional fabric wherein the unidirectional fiber-reinforced fabrichas an assistant thread in a transverse direction and the assistantthread is tangled with the carbon fiber bundle in the longitudinaldirection.

[34] The fiber-reinforced fabric according to [33] above, wherein theassistant thread comprises a low melting point polymer and is adhered tothe carbon fiber bundle in their intersection point through the polymer.

[35] A method of forming a fiber-reinforced plastic, wherein thefiber-reinforced fabric according to any of [32] to [34], wherein thenumber of filaments that compose the carbon fiber bundle is 15000 to100000 or a total fineness of the carbon fiber bundle is 9900 to 65000dtex, is laminated as a fiber base material in at least one or morelayers on a forming die, a medium for diffusing a resin in a surfacedirection is placed thereon, subsequently the fiber base material andthe medium are entirely covered with a bag film, then an inside of thebag film is vacuumized to diffuse a room temperature curable resin onone side of the fiber base material and impregnate the fiber basematerial therewith.

According to the present invention, the carbon fiber bundle which has alarge value of single-fiber fineness and excellent productivity andwhich, despite this, contains few interlaced single fibers therein andhas excellent spreadability, and the carbon-fiber-precursor fibersuitable for the production thereof are provided.

According to the present invention, the carbon-fiber-precursor acrylfiber bundle that can efficiently produce a high quality carbon fiberbundle in which the cross-section double structure of the flame-prooffiber is reduced in high speed burning even when the single fiberfineness is large, and the method of producing the flame-proof fiberbundle using the precursor acryl fiber bundle are provided.

According to the present invention, the carbon fiber bundle having thelarge single fiber fineness and proper physical properties is provided.

According to the present invention, the molding wherein the reduction ofthe compression strength of the molding is small due to increase of thethickness of the molding and the compression strength of the moldingless depends on the thickness while the high flow of the matrix resin iskept is provided.

According to the present invention, it is possible to obtain the carbonfiber bundle, the fiber-reinforced fabric and the fiber-reinforcedplastic, which are excellent in impregnation and have a large towvolume. Thus, processing becomes easy in the uses for the prepreg andthe fabrics that are main uses of the carbon fiber. It is also possibleto produce the carbon fiber composite material having the largersingle-fiber fineness, the higher tensile strength and more excellentintensity reappearance than the conventional carbon fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a step of curing a carbonfiber bundle with a methacryl resin;

FIG. 2 is a cross-sectional view illustrating a carbon fiber bundle(sample) embedded in the methacryl resin;

FIG. 3 is a schematic view illustrating an evaluation method forimpregnation of the carbon fiber bundle with the resin;

FIG. 4 is a view illustrating a method of forming CFRP of the presentinvention;

FIG. 5 is a view illustrating a curing condition of a resin compositionwhen a flow index of the resin composition is measured; and

FIG. 6 is a view illustrating a relation between a strand elasticmodulus and a calorific value per unit mass of the carbon fiber.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

Polyacrylonitrile-Based Copolymer

A content of an acrylonitrile unit in a polyacrylonitrile-basedcopolymer (hereinafter sometimes referred to as a “copolymer”) thatcomposes the carbon-fiber-precursor acryl fiber bundle (hereinaftersometimes referred to as a “precursor fiber bundle) of the presentinvention is 95 to 99 mol %. When the content is 95 mol % or more, thereduction of a copolymerization rate of the acrylonitrile unit does notlead to the reduction of performance of the carbon fiber. On the otherhand, an upper limit of 99 mol % is defined from a required amount of acopolymerization component.

A content of a hydroxyalkyl (meth)acrylate unit in the copolymer is 1 to5 mol %. A carboxylate ester group in the hydroxyalkyl (meth)acrylateunit is pyrolyzed at high temperature of 240° C. or above to become acarboxylate group. When the content of the hydroxyalkyl (meth)acrylateunit in the copolymer is 1 mol % or more, a sufficient effect to promotea flame-proof reaction is obtained when the carboxylate ester group inthe hydroxyalkyl (meth)acrylate unit becomes the carboxylate group inthe flame-proof reaction. On the other hand, when the content is 5 mol %or less, a runaway of the flame-proof reaction can be inhibited.Further, it is possible to inhibit the reduction of a carbonizationyield with dissociation of a hydroxyalkyl group in the flame-proofreaction.

A lower limit of the content of the hydroxyalkyl (meth)acrylate unit ispreferably 1.2 mol % or more in terms of assuring compactness of theprecursor fiber bundle and is more preferably 1.5 mol % or more in thatthe carbon fiber with higher performance can be obtained. The upperlimit of the content of the hydroxyalkyl (meth)acrylate unit ispreferably 4.0 mol % or less in that the runaway of the flame-proofreaction is inhibited and is more preferably 3.0 mol % or less in thatthe reduction of the carbonization yield is inhibited.

Hydroxyalkyl (meth)acrylate that is a raw material of the hydroxyalkyl(meth)acrylate unit includes 2-hydroxyethyl (meth) acrylate,2-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate,monoglyceryl (meth) acrylate, and the like. Further, these monomers maybe used in combination of two or more. When the monomers are combined,if a total amount of the monomers is 5.0 mol % or less, their ratio canbe determined freely.

2-Hydroxyethyl (meth)acrylate is suitable as a component of thecopolymer of the present invention because dissociation temperature of ahydroxyethyl group in the flame-proof reaction is 240° C. or above, itsbulkiness is enough to enhance the oxygen permeability, the mass is lessreduced when the hydroxyethyl group is dissociated and 2-Hydroxyethyl(meth)acrylate is easily industrially available.

Other Monomers

The copolymer of the present invention contains the acrylonitrile unitand the hydroxyalkyl (meth)acrylate unit, but may contain the “othermonomer unit” if necessary.

Vinyl-based monomers copolymerizable with acrylonitrile are preferableas the “other monomer” that is the raw material of the other monomerunit. Specifically, the other monomer includes (meth)acrylate esterssuch as methyl (meth)acrylate, ethyl (meth)acrylate, propyl(meth)acrylate, butyl (meth)acrylate and hexyl (meth)acrylate,halogenated vinyls such as vinyl chloride, vinyl bromide and vinylidenechloride, acids such as (meth)acrylic acid, itaconic acid and crotonicacid and salts thereof, maleic imide, phenyl maleimide,(meth)acrylamide, styrene, α-methylstyrene, vinyl acetate, and the like.These may be used alone or in combination of two or more.

The content of the other monomer in the copolymer of the presentinvention is preferably 3.5 mol % or less in consideration of thecontents of the acrylonitrile unit and the hydroxyalkyl (meth)acrylateunit.

Melting Point Under Heat and Humidity

A melting point under heat and humidity of the copolymer of the presentinvention is preferably 160 to 175° C. When the melting point under heatand humidity is 160° C. or above, adherence between the single fibers inthe precursor fiber bundle can be inhibited and the reduction of qualityand dynamic physical properties of the obtained carbon fiber bundle canbe inhibited. If the melting point under heat and humidity is 175° C. orbelow, for example, when a dry compacted thread during a spinning stepis stretched with steam, the higher steam, i.e., higher pressure steambecomes unnecessary, thus feathering and scratch that occur due to upand down movement of the precursor fiber bundle under the high pressuresteam can be reduced. Thus, the reduction of the quality and the dynamicphysical properties of the obtained carbon fiber bundle can beinhibited.

Water Contact Angle of Copolymer

A water contact angle of the copolymer of the present invention ispreferably 40° or more and 70° or less. When the contact angle of thecopolymer with water is 70° or less, an organic solvent and acoagulation bath solution are exchanged mildly in a spinning neatsolution in a spinning step, particularly in a coagulation step when theprecursor fiber bundle is formed from the copolymer, and thus thecompactness of the precursor fiber bundle is enhanced easily. Also, whenthe contact angle of the copolymer with water is 40° or more,hydrophilicity of the copolymer is kept properly, and the coagulationcan be performed efficiently without causing agglutination betweenadjacent fibers in the spinning step, particularly in the coagulationstep. From these viewpoints, the contact angle of the copolymer with thewater is preferably 55° or more and 65° or less and more preferably 58°or more and 62° or less.

Oxidation Depth De of Copolymer Upon Flame-Proof Treatment

An oxidation depth De upon the flame-proof treatment of a film obtainedfrom the copolymer of the present invention is an indicator of aflame-proof reactivity of the precursor fiber bundle obtained from thecopolymer of the present invention in a burning step, particularly aflame-proof step. That is, the larger the oxidation depth De is, theoxygen is sufficiently diffused inside the single fiber of the precursorfiber bundle and the flame-proof treatment can be evenly performed.Therefore, the oxidation depth De is preferably 4.0 μm or more and 6.0μm or less in terms of oxidation reaction in the flame-proof reaction.

When the oxidation depth De is 4.0 μm or more, the oxygen can bedistributed easily inside the fiber in the flame-proof step in thecarbon-fiber-precursor acryl fiber bundle having the large single-fiberfineness of 2.0 dtex or more and 5.0 dtex or less, high oxygendiffusibility is obtained and the carbon fiber with the high performanceis easily obtained. On the other hand, when the oxidation depth is 6.0μm or less, the progress of the oxidation reaction in the flame-prooftreatment can be easily controlled in an appropriate range, and theyield of the obtained carbon fiber is less reduced. From theseviewpoints, the oxidation depth De is more preferably 4.4 to 5.8 μm, andstill more preferably 4.6 to 5.6 μm. A method of measuring the oxidationdepth De will be described later.

Method of Producing Copolymer

A polymerization initiator is not particularly limited, and azo-basedcompounds, organic peroxide, and redox catalysts such as ammonium saltsof persulfuric acid/sulfurous acid and chloric acid/sulfurous acid canbe used.

In the suspension polymerization, for example, each monomer, distilledwater, ammonium persulfate, ammonium hydrogen sulfide and sulfuric acidare continuously supplied in constant amounts in an overflow typepolymerization container. The mixture is stirred while the temperatureis kept constant, and a polymer slurry that overflowed is washed anddried to obtain the copolymer.

Precursor Fiber Bundle

In the precursor fiber bundle of the present invention, the single-fiberfineness is 1.5 dtex or more and 5.0 dtex or less and the roundness is0.90 or less in the shape of the cross-section perpendicular to thefiber axis of the single fiber. Hereinafter, the precursor fiber bundleis appropriately referred to as a “first group invention” in some cases.

Single-Fiber Fineness

When the single-fiber fineness of the precursor fiber bundle is 1.5 dtexor more, the single fibers are less in contact with one another insidethe precursor fiber bundle. Therefore, the single fibers are lesstangled with one another, and the spreadability of the carbon fiberbundle can be kept when the carbon fiber bundle is made. On the otherhand, when the single fiber fineness of the precursor fiber bundle is5.0 dtex or less, the cross-section double structure does not becomeprominent in the flame-proof step, and the carbon fiber bundle withuniform quality can be produced stably. The single-fiber fineness ismore preferably 2.0 dtex or more and 2.5 dtex or less. The single-fiberfineness is preferably 4.5 dtex or less and more preferably 3.0 dtex orless.

Cross-Sectional Shape

The roundness is 0.90 or less in the cross-sectional shape of the singlefiber in the precursor fiber bundle of the present invention. Thecross-sectional shape is preferably a horsebean shape. When thecross-sectional shape is the horse bean shape having the roundness of0.90 or less, the flame-proof reaction progresses sufficiently withoutshortage of oxygen diffusion inside the single fiber that composes theprecursor fiber bundle upon the flame-proof treatment. As a result, thefeathering in the carbonization step is reduced, the fiber can pass thesteps well, and the strength and the elastic modulus of the obtainedcarbon fiber bundle can be kept properly.

However, when the cross-sectional shape is changed excessively, acontent rate of the fiber cannot be increased when the prepreg isproduced from the obtained carbon fiber bundle, and the dynamic physicalproperty of the composite material is reduced. Thus, the roundness ofthe single fiber that composes the carbon fiber bundle is preferably0.70 or more, more preferably 0.75 or more and still more preferably0.80 or more.

The cross-sectional shape of the single fiber in the precursor fiberbundle of the present invention having the above structure has a shortdistance from the inside to the surface of the fiber. Thus, even whenthe single-fiber fineness is increased to some extent, it is possible toevenly perform the flame-proof treatment, and the carbon fiber bundlewith the high performance is obtained easily.

In the present invention, the roundness is a value determined using thefollowing equation (1), where S and L are a cross-sectional area and acircumferential length, respectively, of the single fiber, which areobtained by observing, under an SEM, the cross-section of the singlefiber perpendicular to the fiber axis of the single fiber and analyzingthe obtained image.

Roundness=4πS/L ²   (1)

Heat quantity Ja, Jb on constant velocity temperature rising exothermiccurve

In the precursor fiber bundle of the present invention, a constantvelocity temperature rising exothermic curve at 30° C. or above and 450°C. or below measured at a temperature rising rate of 10° C./minute inair flow at 100 ml/minute (standard: at 30° C. and 0.10 MPa) using aheat flux type differential scanning calorimeter satisfies the followingconditions. Hereinafter, the precursor fiber bundle is referred to as a“second group invention” in some cases.

Conditions

-   (1) A heat quantity Ja obtained by integrating an exothermic    velocity at 230° C. or above and 260° C. or below of the constant    velocity temperature rising exothermic curve is 100 kJ/kg or more    and 250 kJ/kg or less; and-   (2) Heat quantity Jb obtained by integrating an exothermic velocity    at 260° C. or above and 290° C. or below of the constant velocity    temperature rising exothermic curve is 550 kJ/kg or more and 1050    kJ/kg or less.    The constant velocity temperature rising exothermic curve indicates    the heat quantity generated when the flame-proof reaction progresses    in the precursor fiber bundle.

When the carbon fiber bundle is produced, in the flame-proof step wherethe flame-proofed fiber bundle is obtained from the precursor fiberbundle, a treatment temperature in its early phase is set in the rangeof the temperature that is equal to or higher than the temperature atwhich the flame-proof reaction is initiated and that is equal to orlower than the temperature at which the precursor fiber bundle is notmelted. Meanwhile, when the flame-proof reaction progresses to someextent, the treatment temperature can be raised in order to efficientlyperform the flame-proof reaction. Generally, in order to perform theflame-proof treatment to the precursor fiber bundle at a temperaturezone of 220 to 300° C., the present inventors divided this temperaturezone into two zones that are a flame-proof step first half and aflame-proof step second half using 260° C. as a center, and made acalorific value at 230° C. or above and 260° C. or below a heat quantityJa and a calorific value at 260° C. or above and 300° C. or below a heatquantity Jb, and compared the calorific value in each zone with thequality and the performance of the finally obtained carbon fiber bundle.

As a result, it was found that when the heat quantities Ja and Jb werein the above range, the flame-proof reaction and the oxygen diffusionwere performed in good balance, the cross-section double structure ofthe flame-proofed fiber was inhibited in the flame-proof treatment athigh speed, the carbon fiber bundle with high quality and goodperformance reappearance was obtained efficiently, and the flame-prooftreatment could be evenly given to the precursor fiber bundle having thelarge single-fiber fineness. At that time, the temperature upon theflame-proof treatment was set to the range of 220° C. to 300° C., whichwas the optimal temperature for giving the flame-proof treatment to theprecursor fiber bundle.

That is, when the heat quantity Ja is 100 kJ/kg or more, the flame-proofreaction progresses appropriately in the flame-proof step first half,and the precursor fiber bundle is easily passed through the step withoutbeing melted with heat. When the heat quantity Ja is 250 kJ/kg or less,the flame-proof treatment is easily given to the precursor fiber bundlehaving the large single-fiber fineness without progressing theflame-proof reaction at once in the flame-proof step first half. Theheat quantity Ja is more preferably 120 kJ/kg or more in terms ofenhancement of the productivity by shortening of flame-proof treatmenttime, and is more preferably 200 kJ/kg or less and particularlypreferably 160 kJ/kg or less in that the flame-proof treatment is moreevenly performed to the precursor fiber bundle having the largesingle-fiber fineness.

Meanwhile, when the heat quantity Jb is 550 kJ/kg or more, theflame-proof treatment is easily given to the precursor fiber bundle upto a target density of the flame-proofed fiber without impairing theproductivity in the flame-proof step. When the heat quantity Jb is 1050kJ/kg or less, the flame-proof reaction progresses mildly in theflame-proof step. Thus, the flame-proof treatment is easily and evenlygiven to the precursor fiber bundle having the single-fiber fineness,and the formation of the cross-section double structure is easilyinhibited. The heat quantity Jb is preferably 600 kJ/kg or more in termsof enhancement of the productivity by shortening of flame-prooftreatment time, and is more preferably 700 kJ/kg or more in terms offurther enhancement of the productivity. The heat quantity Jb ispreferably 950 kJ/kg or less in that the flame-proof treatment is moreevenly given to the precursor fiber bundle having the large single-fiberfineness.

From the above, it was found that the heat quantity Ja could be made theindicator of the flame-proof reaction in the flame-proof step first halfand the heat quantity Jb could be made the indicator of the flame-proofreaction in the flame-proof step second half. The heat quantity Ja andthe heat quantity Jb can consistently be made the indicators of theflame-proof reactivity of the precursor fiber bundle. The temperaturezone actually applied to the flame-proof step may or may not include thetemperature zones of the heat quantity Ja and the heat quantity Jb(i.e., 230 to 260° C. and 260 to 290° C.), and can be appropriatelycontrolled in the range of 220 to 300° C. depending on the precursorfiber bundle to be used.

Calorific Value and Half-Value Width of H-NMR Spectra

In the precursor fiber bundle of the present invention, the calorificvalue per unit mass at 215 to 300° C. measured using the heat flux typedifferential scanning calorimeter (hereinafter sometimes abbreviatedas“DSC”) is 3200 to 3800 kJ/kg, and the half-value width of the spectraobserved at 160° C. in solid ¹H-NMR is 10 kHz or more and 14.5 kHz orless. Hereinafter, the precursor fiber bundle is referred to as a “thirdgroup invention” in some cases.

A commercially available heat flux type differential scanningcalorimeter can be used as the heat flux type differential scanningcalorimeter. But, the calorific value is a value obtained under thefollowing measurement conditions.

-   Measurement atmosphere: air-   Gas flow: 100 ml/minute-   Temperature rising condition: 20° C./minute (room temperature to    210° C.), 2° C./minute (210 to 300° C.)    The calorific value is obtained by using a value of a heat flow    quantity at 215° C. as 0 and integrating values of the heat flow    quantity at 215 to 300° C. by time. The calorific value per unit    mass is obtained by dividing the calorific value by a mass of the    sample subjected to the measurement.

When the calorific value is 3200 kJ/kg or more, there are manystructures stable for the heat after the flame-proof step, and theelastic modulus is not reduced when the carbon fiber is made. Thiscalorific value is preferably 3300 kJ/kg or more.

The half-value width of the spectra observed at 160° C. in solid ¹H-NMRis an indicator of molecular movement, where the smaller the value is,the better the molecular movement is. This value is almost the same whenthe composition of the polyacrylonitrile-based copolymer is the same.

When the molecular movement is good, the diffusibility of oxygen isgood. When the half-value width of the spectra is 14.5 kHz or less, thediffusibility of the oxygen is good upon the flame-proof treatment, andthe stable structure can be formed into the fiber bundle even when thesingle-fiber fineness of the precursor fiber bundle is large. Thus, thestrand elastic modulus of the carbon fiber is not reduced and also thestrand strength is not reduced. When the half-value width of the spectrais 10.0 kHz or more, the molecular movement is reduced and anorientation of the molecule is kept easily. The half-value width of thespectra is preferably 10.0 kHz or more and 13.5 kHz or less.

A commercially available apparatus can be used for solid ¹H-NMR, and thehalf-value width of the spectra is a value obtained by measuring by astatic probe in which a coil is secured perpendicularly to a magneticfield.

It is preferable that the precursor fiber bundle that is the “firstgroup invention” further have the feature of the “second groupinvention” or the feature of the “third group invention”.

Method of Producing Precursor Fiber Bundle

The precursor fiber bundle of the present invention can be produced, forexample, by discharging a spinning neat solution containing aconcentration of 15 to 30% by mass of a copolymer obtained by dissolvingthe aforementioned polyacrylonitrile-based copolymer in a solvent into acoagulation bath at a concentration of 30 to 70% by mass at temperatureof 20 to 50° C. to obtain a coagulated thread and stretching thiscoagulated thread with heat and humidity to 2.5 times or more and 6times or less. A spinning method is described below.

Preparation of Spinning Neat Solution

The aforementioned copolymer is dissolved in the solvent by the knownmethod to use as a spinning neat solution. An organic solvent such asdimethylacetamide, dimethylsulfoxide and dimethylformamide, and anaqueous solution of an inorganic compound such as zinc chloride andsodium thiocyanate can be used as the solvent. The organic solvent ispreferred because no metal is contained in the precursor fiber and thestep is simplified, and of these, it is preferable to usedimethylacetamide because the compactness of the coagulated thread and astretched thread with heat and humidity is high.

Coagulation

It is preferred that the spinning neat solution have the concentrationof the copolymer to some extent or more so as to obtain compactcoagulated threads and have the appropriate viscosity and fluidity. Theconcentration of the copolymer in the spinning neat solution ispreferably in the range of 15 to 30% by mass and more preferably in therange of 18 to 25% by mass.

A known method can be employed as the spinning method, and specificallya wet spinning method, a dry wet spinning method, a dry spinning methodand the like are included. Of these, the wet spinning method ispreferably used in terms of productivity.

The coagulated thread is obtained by discharging the above spinning neatsolution into the coagulation bath through a spinning nozzle. It ispossible to control the roundness of the single fiber in the precursorfiber bundle in the coagulation step in the spinning step. Theconcentration is preferably 30% by mass or more and 60% by mass or lessand the temperature is preferably 20° C. or above and 40° C. or below asa coagulation bath condition. When the coagulation bath condition is inthis range, the precursor fiber bundle having the roundness of 0.75 ormore and 0.90 or less can be obtained while the proper coagulation rateis kept.

When the coagulation bath concentration is 60% by mass or less, anexchange rate of the solvent and water on the surface of the spinningneat solution discharged into the coagulation bath is above a diffusionrate of the water into the spinning neat solution, the roundness of theprecursor fiber bundle is kept in the above range as well as the compactprecursor fiber can be obtained. Further, the adhesion between singlethreads in the precursor fiber bundle can be inhibited. In particular,when the precursor fiber bundle having both the large single-fiberfineness and the large total fineness is spun, the concentration ispreferably 55% by mass or less in terms of further inhibiting theadhesion between the single threads. When the coagulation bathconcentration is 30% by mass or more, the exchange rate of the solventand water on the surface of the spinning neat solution discharged intothe coagulation bath being prominently above the diffusion rate of thewater into the spinning neat solution can be inhibited, the roundness ofthe precursor fiber bundle is kept in the above range in the range inwhich rapid shrinkage of the coagulated thread does not occur, and thecompact precursor fiber can be obtained.

Meanwhile, when the coagulation bath temperature is 40° C. or below, theexchange rate of the solvent and water on the surface of the spinningneat solution discharged into the coagulation bath being prominentlyabove the diffusion rate of the water into the spinning neat solutioncan be inhibited, the roundness of the precursor fiber bundle is kept inthe above range in the range in which rapid shrinkage of the coagulatedthread does not occur, and the compact precursor fiber can be obtained.Also when it is 20° C. or above, the exchange rate of the solvent andwater on the surface of the spinning neat solution discharged into thecoagulation bath and the diffusion rate of the water into the spinningneat solution are kept appropriately so that it becomes possible tostably produce the precursor fiber bundle. Further, it is not necessaryto excessively cool the coagulation bath, capital investment and runningcost can be reduced, and it becomes possible to produce the precursorfiber bundle with low cost.

When the compactness or the homogeneity of the fiber structure in theprecursor fiber bundle is insufficient, a site of such a fiber structurebecomes a defect point upon burning, and the performance of the carbonfiber is sometimes impaired. In order to obtain the compact andhomogenous precursor fiber bundle, the characteristics of the coagulatedthread is very important, and it is preferred that less than onemacrovoid be present in a length of 1 mm of the precursor fiber. Here,the macrovoid is a collectively termed void having a spherical, spindleor cylindrical shape having a size of 0.1 to several μm in maximumdiameter.

The coagulated thread of the present invention does not have such amacrovoid and is obtained by sufficient and uniform coagulation. Whenthere are many macrovoids, the coagulated thread is devitrified tobecome cloudy, but the macrovoid is scarcely present in the coagulatedthread of the present invention, which thus is not devitrified and doesnot become cloudy. The presence or absence of the macrovoid can easilybe determined by directly observing the coagulated thread under anoptical microscope or cutting the coagulated thread by an appropriatemethod and observing its cut surface under the optical microscope.

Stretching

Subsequently, the obtained coagulated thread is stretched under heat andhumidity. This can further enhance the orientation of the fibers. Thestretching under heat and humidity is specifically performed bystretching of the coagulated thread while washing with water orstretching in hot water. The stretching simultaneously while washingwith water is preferred in terms of simplification and efficiency of thespinning step, and the stretching in the hot water is preferred in termsof productivity. A stretching magnification in the stretching under heatand humidity is preferably 2.5 times or more and more preferably 3 timesor more. When the magnification is lower than 2.5 times, the effect ofenhancing the orientation of the fiber easily becomes insufficient. Theupper limit of the stretching magnification is not particularly limited,and is preferably 6 times or less in terms of stability of the spinningstep.

Further, a silicon-based oil agent addition treatment is given to thefiber bundle after finishing the stretching under heat and humidity. Forexample, the common silicon-based oil agent such as an aminosilicone-based oil agent can be used as the silicon-based oil agent. Thesilicon-based oil agent is prepared at a concentration of 0.4 to 1.5% bymass for the use. The range of the concentration of the silicon-basedoil agent is preferably 0.8 to 1.5 by mass.

Subsequently, the fiber bundle after finishing the silicon-based oilagent addition treatment is dried. The obtained dried and compactedthread is further stretched to 1.2 to 4 times by steam stretching or dryheat stretching. The stretching magnification is 1.2 times or more andpreferably 1.3 times or more.

Interlace Treatment

Subsequently, a water percentage in the fiber bundle after the steamstretching or the dry heat stretching is adjusted by touch roll ifnecessary, then an interlace treatment is performed by blowing the airby a known method to obtain the precursor fiber bundle. The interlacetreatment is not essential in the present invention, but by impartingthe interlace to the filaments in the precursor fiber bundle, it ispossible to obtain a fiber bundle keeping a form of one tow withimparting a bundle integrity. Also, by making it difficult to unravelthe fiber bundle, the passing of the burning step can be enhanced.

The water percentage before the interlace treatment is performed ispreferably 15% by mass or less, more preferably 10% by mass or less, andstill more preferably 3 to 5% by mass. When the water percentage exceeds15% by mass, when the interlace is performed by blowing the air to thefiber bundle, the single fiber becomes difficult to be interlaced. Thewater percentage herein is a value obtained by the following equation.

Water percentage (% by mass)=(W−W ₀)×100/W ₀

-   W: Mass of wet fiber bundle-   W₀: Mass after drying the wet fiber bundle at 105° C. for 2 hours in    a hot wind dryer.

An interlace degree in the precursor fiber bundle to which the interlacetreatment was performed is preferably in the range of 5 to 20/m and morepreferably in the range of 10 to 14/m. When the interlace degree is 5/mor more, the fiber bundle is difficult to be dissected out and thepassing of the burning step is good. When the interlace degree is 20/mor less, impregnation with the resin and fiber spreading are good in theobtained carbon fiber bundle.

The interlace degree in the precursor fiber bundle herein is a parameterindicating how many times per fiber length of 1 m one single fiber ofthe fiber bundle is interlaced with another adjacent single fiber. Theinterlace degree is measured by a hook drop method.

Flame-Proof Treatment

Subsequently, the method of producing the carbon fiber of the presentinvention will be described. First, the flame-proof treatment isperformed to the precursor fiber bundle under an oxidation atmosphere attemperature of 240° C. or above and 300° C. or below for 90 minutes orless to make a flame-proofed fiber bundle. “Under the oxidationatmosphere” in the present invention means in air containing anoxidizing substance such as nitrogen dioxide, sulfur dioxide and oxygen.

Flame-Proof Treatment Temperature

When the temperature in the flame-proof treatment is 240° C. or above,the flame-proof treatment can be performed efficiently without causingthe runaway of the flame-proof reaction. When the temperature is 300° C.or below, it is possible to perform the flame-proof treatment withoutthermally decomposing a polyacrylonitrile skeleton in the precursorfiber, and the treatment time for 90 minutes or less can increase thedensity of the flame-proofed fiber bundle to 1.35 to 1.43 g/cm³.

The temperature in the flame-proof treatment is preferably 250° C. orabove in that the flame-proof treatment time is shortened and ispreferably 280° C. or below in that the runaway of the flame-proofreaction is inhibited.

Flame-Proof Treatment Time

Flame-Proof Treatment Time

The flame-proof treatment time is preferably 10 to 90 minutes. When theflame-proof treatment time is 10 minutes or more, the oxygen can bediffused sufficiently inside the single fiber that composes theprecursor fiber bundle. When the flame-proof treatment time is 90minutes or less, it is possible to efficiently produce the carbon fiberbundle without impairing the productivity by the flame-proof treatmentstep in the production step of the carbon fiber bundle. Further, theflame-proof treatment time is more preferably 30 to 70 minutes in termsof enhancing the performance and the productivity of the carbon fiberbundle.

Density of Flame-Proofed Fiber Bundle

The density of the flame-proofed fiber bundle obtained by theflame-proof treatment is preferably 1.35 to 1.43 g/cm³. When the densityis 1.35 g/cm³ or more, it is possible to produce the carbon fiberwithout reducing the yield of the carbon fiber bundle. Generally, it isknown that the higher the density of the flame-proofed fiber is, theyield of the carbon fiber bundle is further enhanced, but theperformance of the carbon fiber is reduced. When the density of theflame-proofed fiber bundle is 1.43 g/cm³ or less, it is possible toenhance the yield of the obtained carbon fiber bundle while thereduction of the performance of the carbon fiber is inhibited. Thedensity of the flame-proofed fiber bundle is more preferably 1.38 to1.41 g/cm³ in terms of keeping the performance of and enhancing theyield of the obtained carbon fiber.

Flame-Proof Behavior of Hydroxyalkyl (Meth)Acrylate

In the step of performing the flame-proof treatment to the precursorfiber bundle of the present invention, the progress of the flame-prooftreatment is inhibited while the hydroxyalkyl carboxylate group(carboxylate ester group) in the hydroxyalkyl (meth)acrylate unit isthermally decomposed to become the carboxylate group. This enables toassure a time enough to diffuse the oxygen inside the single fiber andsubsequently perform the flame-proof treatment rapidly from the hightemperature at 240° C. or above when the thermal decomposition of thehydroxyalkyl carboxylate group in the methacrylate hydroxyalkyl unitoccurs to make the carboxylate group.

Further, the hydroxyalkyl carboxylate group in the hydroxyalkyl(meth)acrylate unit is a relatively bulky functional group, and has theeffect to improve the oxygen permeability in the flame-proof step. Theoxygen is efficiently diffused inside the single fiber by these effectswhile the progress of the flame-proof reaction is inhibited. Thus, evenwhen the flame-proof treatment of the precursor fiber bundle having thelarge single-fiber fineness is started from the high temperature andperformed in a short period of time, the formation of the cross-sectiondouble structure is inhibited and the flame-proofed fiber with uniformflame-proof progress degree can be obtained.

Pre-Carbonization Treatment

Pre-Carbonization Treatment

After the flame-proof treatment and before the carbonization treatment,it is also possible to perform a pre-carbonization treatment in whichthe obtained flame-proofed fiber bundle is treated in an inert gas attemperature of 550° C. or above and 800° C. or below as the highesttemperature.

Carbonization Treatment

The carbon fiber bundle can be produced by performing the carbonizationtreatment to the obtained flame-proofed fiber bundle in the inert gas attemperature of 800° C. or above and 2000° C. or below. A graphite fibercan also be produced by further treating this carbon fiber in the inertgas at high temperature of 2500° C. or above and 2800° C. or below. Inthe carbon fiber bundle obtained by the carbonization treatment, thediameter of the single fiber is 8 μm or more, and the roundness is 0.90or less in the cross-sectional shape perpendicular to the fiber axis ofthe single fiber. It is preferred that the cross-sectional shape be thehorsebean shape.

Diameter Di (Maximum Feret Diameter) of Single Fiber in Carbon FiberBundle

The cross-section perpendicular to the fiber axis of the single fiber isobserved under a scanning electron microscope (SEM), and a major axis(maximum feret diameter) of the cross-section on the obtained image ismeasured using image analysis software (product name: Image-ProPLUSmanufactured by Nippon Roper K.K.). The mean value of major axes ofthese cross-sections is used as the diameter Di. The diameter Di ispreferably 8 to 20 μm and particularly preferably 10 to 15 μm. A methodof measuring the diameter Di will be described later.

The carbon fiber bundle obtained by the production method of the presentinvention is composed of the single fibers having the diameter Di of 8μm or more. Thus, bending rigidity of each single fiber is high, and thetangle between fibers due to disturbance upon production steps occursscarcely. Thus, an interlace number in the fiber bundle is reduced.Further, when the single fiber is thick, the single fibers are less incontact with one another in the fiber bundle and friction resistancebetween the single fibers is reduced. Thus, the carbon fiber bundle hasvery good spreadability even when the number of the fibers is large.Thus, the diameter Di is more preferably 9 μm or more and still morepreferably 10 μm or more. But, when the diameter of the carbon fiber isthick, although the problem of the oxygen permeability described lateris solved, an existence probability of the defect is increased inproportion to the increase of a volume per unit length. Thus, thestrength of the carbon fiber is reduced. To not reduce the strength ofthe carbon fiber, the diameter Di is preferably 17 μm or less and morepreferably 15 μm or less.

Cross-Sectional Shape of Single Fiber in Carbon Fiber Bundle

The cross-sectional shape of the single fiber in the carbon fiber bundleobtained by the production method of the present invention isrepresented by the roundness of the cross-section perpendicular to thefiber axis of the single fiber in the carbon fiber bundle. The roundnessis defined by the equation (1) in the same manner as in the roundness ofthe precursor fiber bundle.

The roundness is 0.70 or more and 0.90 or less in the cross-sectionalshape of the single fiber in the carbon fiber bundle obtained by theproduction method of the present invention. The cross-sectional shape ismore preferably the horsebean shape. By making the cross-sectional shapethe horsebean shape having the roundness of 0.70 or more and 0.90 orless, which is a relatively simple shape, the oxygen is diffused in fullmeasure inside the single fibers that compose the precursor fiber bundlein the flame-proof treatment and the flame-proof reaction progressessufficiently. Consequently, the feathering in the carbonization step isreduced, a step passing is good, and the strength and the elasticmodulus of the obtained carbon fiber bundle can be kept properly. Thecarbon fiber of the present invention having the roundness of 0.70 ormore and 0.90 or less can keep the higher value of the strand strengththan the carbon fiber having an almost round cross-sectional shapehaving the roundness of more than 0.9, even when the single-fiberfineness becomes large. The single fibers can also be packed tightly.Thus, a fiber content rate in the prepreg is enhanced and the dynamicproperty of the composite material can be enhanced.

Further, a 0° compression strength when a unidirectional prepreg islaminated to make a composite panel exhibits a higher value in the caseof using the carbon fiber having the roundness of 0.70 or more and 0.90or less than in the case of using the carbon fiber having the almostround cross-sectional shape having the roundness of more than 0.9. Whenthe distance from the surface to the center in the single fiber isshort, the flame-proof treatment is performed evenly even when thesingle-fiber fineness is made relatively large. Thus, the roundness ofthe single fiber that composes the carbon fiber bundle is morepreferably 0.88 or less and most preferably 0.86 or less. However, ifthe cross-sectional shape is changed excessively, the fiber content ratewhen the prepreg is produced cannot be increased and the dynamicproperty of the composite material is reduced. Thus, the roundness ofthe single fiber that composes the carbon fiber bundle is preferably0.75 or more and more preferably 0.80 or more.

Meanwhile, as described in Japanese Unexamined Patent Application,Publication No. H11-124743, the single fibers are tangled with oneanother and the spreadability is reduced in the carbon fiber having arelatively simple variant cross-section such as flatness and threeleaves. In the single fiber having a complicated variant cross-sectionsuch as eight leaves and C type, the single fibers are less tangled withone another, but conversely it becomes impossible to pack the singlefibers tightly, the fiber content rate when the prepreg is producedcannot be increased, and the dynamic property of the composite materialis reduced.

Surface Morphology of Carbon Fiber

In the carbon fiber bundle of the present invention, it is preferredthat a wrinkle extending in a lengthwise direction of the fiber bepresent on the surface of the carbon fiber. The wrinkle extending in thelengthwise direction of the fiber plays a very important role inappearance of a mechanical property of a fiber-reinforced resin materialusing the carbon fiber as a reinforcing material. Because, this isdirectly involved in the formation of an interface phase between thecarbon fiber and the resin and characterizes one of three elements, thefiber, the matrix resin and the interface phase that compose thefiber-reinforced resin material. The wrinkle on the surface of thesingle fiber refers to a concavo-convex form having a certain length orlonger in a certain direction. Here, the certain length or longer refersto a length of about 0.6 μm to 1.0 μm. Its direction is not particularlylimited, and may be parallel to or perpendicular to or may have someangle to the direction of the fiber axis. Due to the common method ofproducing the carbon fiber bundle, the wrinkle nearly parallel to thedirection of the fiber axis is present on the surface of the commoncarbon fiber.

It is preferred that the carbon fiber bundle of the present inventionhas a plurality of groove-shaped concavo-convex parts extending to thelengthwise direction of the single fiber on the surface of the singlefiber and the difference in height between the highest and the lowestparts (depth of wrinkle) be 80 μm or less in the range of 2 μm of acircumference length in the single fiber. When the depth of the wrinklebecomes too deep, the bundle integrity of the fiber bundle is reduced,passing through the burning step when the carbon fiber bundle isproduced is deteriorated, and the carbon fiber bundle cannot be obtainedstably. Also, a surface defect of the carbon fiber bundle is increased,and the strand strength is reduced. When the depth of the wrinkle isshallow, the impregnation is likely deteriorated, but the impregnationis enhanced in the present invention by utilizing the carbon fiberbundle having the large fineness. The depth of the wrinkle on thecross-section in the precursor fiber bundle and the carbon fiber bundleis determined by changing the coagulation bath concentration andtemperature and further the stretching condition.

Performance of Carbon Fiber Bundle

In the carbon fiber bundle of the present invention, the strand tensilestrength is preferably 3000 MPa or more. When the strand tensilestrength is excessively low, such a fiber bundle can scarcely be used inmost fields where the carbon fiber is currently used as a structuralmaterial. Thus, such a tensile strength is more preferably 3500 MPa ormore. When it is 4000 MPa or more, the carbon fiber can be applied toexisting most fields for industrial uses including wind mills,automobiles and building materials.

In the carbon fiber bundle of the present invention, the strand tensileelastic modulus is preferably 200 GPa or more. When the strand tensileelastic modulus is excessively low, such a fiber bundle can scarcely beused in most fields where the carbon fiber is currently used as astructural material. Thus, the strand tensile elastic modulus is morepreferably 210 GPa or more. When it is 220 GPa or more, the carbon fibercan be applied to the most existing fields.

In the carbon fiber bundle of the present invention, the total finenessis preferably 30000 dtex to 90000 dtex. When the total fineness is inthis range, the carbon fiber bundle is suitable for the production ofcomposites/moldings in large size. When the total fineness is increasedby combining a plurality of thin fiber bundles, gaps between the fiberbundles occur, and it is difficult to produce the composites/moldings inlarge size having the high quality. When the total fineness is 30000dtex or more, the productivity can be increased and the production costis easily reduced, which is thus preferable. When the total fineness is90000 dtex or less, the handling is easy. Thus, the total fineness ismore preferably 60000 dtex or less and still more preferably 40000 dtexor less in terms of this point.

Surface Treatment of Carbon Fiber Bundle

In the carbon fiber bundle of the present invention, a surface treatmentmay be performed before a sizing treatment step. For example, it ispreferred that affinity and adhesiveness between the carbon fiber andthe matrix resin in the composite material be enhanced by performing anelectrolytic oxidation in an electrolytic solution or giving oxidationin a gas phase or a liquid phase.

Sizing Treatment Step

A sizing treatment and a drying treatment are performed in a step ofperforming a sizing treatment to the carbon fiber bundle. The method forthe sizing treatment is not particularly limited, and a desired sizingagent may be given to the carbonated fiber bundle. For example, a rollersizing method, a roller immersion method, a spraying method, and thelike can be included.

A sizing treatment solution that can be used for the step of giving thesizing treatment to the carbon fiber bundle of the present invention isnot particularly limited, and those having the property suitable forhigher processing can be selected. For example, in order to evenlyimpregnate a line of threads, a solution containing the sizing agent ismade into an emulsion or a suspension, and this may be adhered to thecarbonated fiber bundle, which may be then dried in a drying apparatusto remove the solvent or the dispersion medium.

A major ingredient of the sizing agent in the sizing treatment solutionincludes, but is not particularly limited to, epoxy resins,epoxy-modified polyurethane resins, polyester resins, phenol resins,polyamide resins, polyurethane resins, polycarbonate resins, polyetherimide resins, polyamide imide resins, polyimide resins, polyimideresins, bismaleimide resins, urethane-modified epoxy resins, polyvinylalcohol resins, polyvinyl pyrrolidone resins, polyether sulfone resins,and the like.

The content of the sizing agent in the sizing treatment solution is notparticularly limited, but is preferably 0.2 to 20% by mass and morepreferably 3 to 10% by mass. By making the content of the sizing agentin the sizing treatment solution 0.2% by mass or more, it is possible tosufficiently impart the function that is desired to the carbon fiber.Also, by making the content of the sizing agent in the sizing treatmentsolution 20% by mass or less, an amount of the adhered sizing agentbecomes proper, and the impregnation of the carbon fiber bundle with thematrix resin when the composite material is produced, which is apost-step, becomes good.

The solvent or the dispersion medium used for the sizing treatmentsolution is not particularly limited, but it is preferable to use waterin terms of handling and safety.

The amount of the adhered sizing agent is preferably 0.3 to 5% by massand more preferably 0.4 to 3% by mass based on 100% by mass of thecarbon fiber bundle. By making the amount of the adhered sizing agent0.3% by mass or more, it is possible to sufficiently impart the functionthat is desired to the carbon fiber. Also, by making the amount of theadhered sizing agent 3% by mass or less, the impregnation of the carbonfiber bundle with the matrix resin when the composite is produced, whichis the post-step, becomes good.

In the drying treatment after the sizing treatment, the solvent or thedispersion medium in the sizing treatment solution is dried and removed.A condition at that time is suitably in the range at temperature of 120to 300° C. for 10 seconds to 10 minutes and more suitably in the rangeat temperature of 150 to 250° C. for 30 seconds to 4 minutes. When thedrying temperature is 120° C. or above, the solvent can be removedsufficiently. When the drying temperature is 300° C. or below, thequality of the carbon fiber bundle to which the sizing treatment wasperformed can be kept.

The method for the drying treatment is not particularly limited, and forexample, a method of drying by contacting the carbon fiber bundle to ahot roll using a vapor steam as a heat source, and a method of dryingthe carbon fiber bundle in an apparatus in which hot wind is circulatedcan be included.

Unidirectional Fiber-Reinforced Fabric

The carbon fiber bundle of the present invention can be used suitablyfor the following unidirectional fiber-reinforced fabric. In theunidirectional fiber-reinforced fabric of the present invention, aweight per unit when the carbon fiber bundle is arranged in thelongitudinal direction is preferably 300 to 1000 g/m². Generally, whenthe weight per unit of the fiber fabric is as small as about 200g/m², aninter-fiber void becomes large. Thus, the impregnation with the resinbecomes good. On the other hand, when the weight per unit of the fiberfabric is large, the inter-fiber void becomes small, the fluidity of theresin becomes worse, impregnation failure occurs and the impregnationtakes a long time. In addition, when a molding processing is performed,a small number of laminations by the fabric having the large weight perunit of the fiber fabric can reduce the cost compared with a largenumber of the lamination by the fabric having the small weight per unitof the fiber fabric. Thus, in the molding processing that requires thelamination, it is advantageous to use the fiber fabric having the weightper unit that is large as possible. By the use of the carbon fiberbundle of the present invention, it is possible to obtain the fiberfabric where the impregnation with the resin is good and theimpregnation for a long time is not required even when the weight perunit of the fiber fabric is in the range of 300 to 1000 g/m².

The method of obtaining the fabric having the large weight per unit ofthe fiber fabric is generally broadly classified into two. One of themis a method of obtaining the fabric having the large weight per unit byusing the carbon fiber bundle commonly used and having 12,000 filamentsand increasing a weave density of the fabric. The second one is a methodof obtaining the fabric using the carbon fiber bundle having 48,000 ormore filaments. However, in order to obtain the fabric having the largeweight per unit, it is much easier in terms of passing the step that thefabric is woven using the carbon fiber bundle having many filaments,which is referred to as a so-called large tow. When any of thesefiber-reinforced fabrics is used as a fiber base material, it isnecessary to use a large number of carbon fiber bundles when the carbonfiber bundle having a small number of filaments is used. Also when thecarbon fiber bundle having a large number of filaments is used, althoughthe number of carbon fiber bundles used is small, the number of thefilaments is large. Thus, in any of these cases, eventually theinter-fiber void becomes small, the fluidity of the resin upon moldingprocessing becomes worse, impregnation failure occurs and theimpregnation takes a long time.

In order to solve them, by using the carbon fiber bundle of the largetow and having the single fiber with the large diameter as thefiber-reinforced fabric, the fluidity of the resin becomes good and theimpregnation time is largely shortened. A reason for this is that theinter-fiber void becomes large to improve the fluidity of the resin byincreasing the diameter of the single fiber. Further in the presentinvention, the carbon fiber bundle previously defined and having theroundness of 0.7 or more and 0.9 or less in the fiber cross-section ofthe single fiber is preferred. When the roundness exceeds 0.9, thebundle integrity tends to increase excessively. When the bundleintegrity increases excessively, it becomes difficult to dissect out thesingle fiber evenly. That is, the impregnation with the resin is reduceddue to the decreased inter-fiber voids. On the other hand, when theroundness is less than 0.7, the bundle integrity is reduced, the burningstep when the carbon fiber bundle is produced is deteriorated, and thecarbon fiber bundle cannot be produced stably. Thus, by using the singlefiber having the roundness of 0.7 or more and 0.9 or less, the bundleintegrity of the carbon fiber bundle can be controlled appropriately, abalance between the bundle integrity and easiness to be dissected isexcellent, and the impregnation with the resin upon molding processingis also enhanced.

The fabric used for the present invention is the unidirectionalfiber-reinforced fabric where the carbon fiber bundle is arranged in thelongitudinal direction, and an assistant thread is used in a transversedirection. A basic structure of this fabric is already well-known forthe use for earthquake-proof reinforcing materials. The line of threadhaving a smaller fineness than that of the carbon fiber bundle used inthe longitudinal direction is typically used as the assistant thread inorder to enhance the strength of the mechanical physical property of thecomposite. That is, the carbon fiber bundle arranged in the longitudinaldirection and the assistant thread arranged in the transverse directionare always complexed alternatively one by one. Thus, flexion occursregardless of the size of the carbon fiber bundles and this impairs theintensity reappearance. The degree of the flexion is proportional to thefineness of the assistant thread. Thus, the thicker the assistant threadis, the larger the flexion of the carbon fiber bundle is. The mechanicalphysical property is also largely reduced. Thus, the assistant thread ispreferably thin as possible, but the assistant thread is notparticularly limited as long as the morphology of the fabric is keptagainst an external force.

Glass fibers are often used for the assistant thread in general, but thematerials are not limited thereto. Also typically, the number of theassistant threads to be used in the unidirectional fabric is 10/inch orless that is relatively few in consideration of handling of the fabric,and the thin assistant thread and a warp thread are complexed. Thus,there is no binding force and the handling of the fabric is very poor.Thus, the assistant thread containing a polymer having a low meltingpoint is used, and the binding force is kept by adhering the carbonfiber and the assistant thread at their intersection point through thepolymer. Thermally fused fibers having the low melting point such asnylon and polyethylene are used as the polymer fiber having the lowmelting point. When the assistant thread and thermally fused fiber aremade into a complexed thread for covering, twisted yarn and paralleledadhesion, there is no problem. Further, a method of utilizing a heatroll or a method of utilizing radiation heat such as far infrared heatermay be used as an adhesion method.

Method of Forming CFRP

The method of forming CFRP of the present invention is described usingFIG. 4. In FIG. 4, a mold releasing agent is applied onto a forming die11, and the carbon fiber fabric 12 of the present invention is laminatedthereon in predetermined layers in a predetermined direction as a fiberbase material. Further, Peel ply 15 is laminated thereon, and a medium14 is placed thereon to diffuse a resin on a surface of the fiber basematerial. Also, a spiral tube 13 for depositing the resin is placed onboth ends in a fiber axis direction of the fiber base material, and asuction spout 18 of a vacuum pump is attached. These are entirelycovered with a bag film 16, and a circumference of the bag film 16 isadhered to the forming die 11 with a sealing material 17 so that the airdid not leak. A discharge spout 20 of the resin injected from a resintank is linked to the spiral tube 13. A thermally curable resin that isa syrup and curable at ambient temperature, to which a curing agent in apredetermined amount was added, has been placed in the resin tank (notshown in figure). The impregnation with the resin is largely affected bya viscosity of the resin to be used. A low viscosity article having thegood fluidity of the resin is typically used for RTM molding and vacuumbag molding. The viscosity upon injection of the resin is preferably 500mPa·s or less and more preferably 300 mPa·s or less.

Then the fiber base material covered with the bag film 16 is vacuumizedto a vacuum pressure of about 70 to 76 cmHg using the vacuum pump, andthen a valve 19 is opened and the resin is injected. An inside coveredwith the bag film 16 is a vacuum state, and a flow resistance of theresin is smaller in a surface direction of the medium than in athickness direction of the fiber base material. Thus, the resin is firstdiffused to the surface of the medium and then the impregnationprogresses to the thickness direction of the fiber base material.However, this impregnation degree is considerably affected by themorphology of the carbon fiber fabric 12 used as the fiber basematerial. Of course, when the fabric has the voids between the lines offiber threads, the impregnation with the resin to the thicknessdirection is completed more rapidly. When a monofilament of polyethyleneor polypropylene having a fiber diameter of 0.2 to 0.5 mm is used as themedium, it is possible to use sheets formed with mesh tone and raschelknitting, and there is no restriction. It is preferred that the vacuumpump be driven at least until the impregnation with the resin iscompleted and the inside of the bag film be kept in a vacuum state.After curing the resin, a CFRP molded article is obtained by peelingPeel ply 15, removing the medium 14 and the bag film 16, and releasingfrom the forming die.

It is necessary that the resin can pass through Peel ply, and nylonfiber fabrics, polyester fiber fabrics or glass fiber fabrics can beused. The smaller the weave density of the fabric is, the larger thespace is. Thus, the resin passes easily, but concavo-convex patternsoccur on the surface of the fiber base material when the resin is curedand finally peeled. Thus, it is better to select those where the resinpasses well as possible and the concavo-convex pattern hardly occurs onthe surface. The bag film is necessary to be gas-tight, and a nylonfilm, a polyester film or the like is used.

Carbon Fiber Prepreg

The present invention also relates to a carbon fiber prepreg composed ofthe carbon fiber bundle and the matrix resin. The carbon fiber in thecarbon fiber prepreg of the present invention is not particularlylimited, and includes PAN-based carbon fibers, PITCH-based carbonfibers, and the like. The PAN-based carbon fiber is desirable. Thosehaving the single-fiber fineness of 1.2 to 2.4 dtex are used, and inparticular the carbon fiber bundle of the present invention is suitablyused. When the single-fiber fineness is 1.2 dtex or more, a compressionstrength keeping rate is increased when the thickness of the molding isincreased. When the single-fiber fineness is 2.4 dtex or less, amechanical strength of the molding is good. One type of the carbon fiberbundle may be used or multiple types thereof may be aligned regularly orirregularly and used in the same prepreg. Typically, a single directionprepreg is the most suitable for the use where a high specific strengthand a high specific elastic modulus are required in the certaindirection, but it is possible to use those which are previouslyprocessed into a sheet form such as a long fiber mat and a fabric.

Matrix

The matrix resin is not particularly limited, but a flow index thereofis preferably 5000 Pa⁻¹ or more. The matrix resin includes epoxy resins,polyester resins, phenol resins, polyimide resins, maleimide resins,resins having an acetylene end, resins having a vinyl end, resins havinga cyanate ester end, and the like. The epoxy resin is preferred.

The “flow index” is as follows. The viscosity of the resin compositionis measured using VAR-100 (manufactured by Rheometrics) under thecondition of gap: 0.5 mm, measurement frequency: 10 rad/sec, stress: 300dyne/cm², measurement times; every 30 seconds. A temperature conditionwas set in the same manner as in the curing condition (FIG. 5), and themeasurement was terminated when the viscosity of the resin compositionwas increased by two digits from the lowest viscosity. The flow index isdefined by the following equation. But, η: viscosity, t: time, andmeasurement values at n times are ηn and tn.

$\begin{matrix}{{{Flow}\mspace{14mu} {index}} = {\sum\limits_{i = 1}^{n}{\left( {{1/\eta_{i}} + {1/\eta_{i - 1}}} \right){\left( {t_{i} - t_{i - 1}} \right)/2}}}} & (2)\end{matrix}$

Any epoxy resin can be used as the epoxy resin. Any epoxy resin can becombined depending on the purpose, for example, a polyfunctional epoxyresin or an epoxy resin having a rigid ring structure in its backbonecan be combined for increasing heat resistance, or a low molecular epoxyresin or an alicyclic epoxy resin can be combined for decreasing theviscosity of the resin composition. For example, a compound having ahydroxyl group in its molecule and a glycidyl ether-type epoxy resinobtained from epichlorohydrin, a compound having an amino group in itsmolecule and a glycidyl amine-type epoxy resin obtained fromepichlorohydrin, a compound having a carboxyl group in its molecule anda glycidyl ester-type epoxy resin obtained from epichlorohydrin, analicyclic epoxy resin obtained by oxidizing a compound having a doublebond in its molecule, or an epoxy resin where two or more groupsselected therefrom are present together in its molecule, and the likeare used.

Specific examples of the glycidyl ether-type epoxy resin includebisphenol A-type epoxy resins, bisphenol F-type epoxy resins,resorcinol-type epoxy resins, phenol novolac-type epoxy resins, othertrisphenol novolac-type epoxy resins, polyethylene glycol-type epoxyresins, polypropylene glycol-type epoxy resins, naphthalene-type epoxyresins, dicyclopentadiene-type epoxy resins, and regioisomers thereofand those obtained by substituting them with an alkyl group or halogen.

Commercially available products of the bisphenol A-type epoxy resinsinclude EPON825, jER826, jER827, jER828 (manufactured by MitsubishiChemical Corporation), Epiclon 850 (manufactured by DIC), EpotohtoYD-128 (manufactured by Nippon Steel Chemical Co., Ltd.), DER-331,DER-332 (manufactured by Dow Chemical), Bakelite EPR154, BakeliteEPR162, Bakelite EPR172, Bakelite EPR173, and Bakelite EPR174(manufactured by Bakelite AG).

Commercially available products of the bisphenol F-type epoxy resinsinclude jER806, jER807, jER1750 (manufactured by Mitsubishi ChemicalCorporation), Epiclon 830 (manufactured by DIC), Epotohto YD-170,Epotohto YD-175 (manufactured by Nippon Steel Chemical Co., Ltd.),Bakelite EPR169 (manufactured by Bakelite AG) and GY281, GY282, GY285(manufactured by Huntsman Advanced Materials), and the like.

Commercially available products of the resorcinol-type epoxy resinsinclude Denacol EX-201 (manufactured by Nagase ChemteX Corporation), andthe like.

Commercially available products of the phenol novolac-type epoxy resinsinclude jER152, jER154 (manufactured by Mitsubishi ChemicalCorporation), Epiclon 740 (manufactured by DIC), EPN179, EPN180(manufactured by Huntsman Advanced Materials), and the like.

Trisphenol novolac-type epoxy resins include Tactix742 (manufactured byHuntsman Advanced Materials), EPPN501H, EPPN501HY, EPPN502H, EPPN503H(manufactured by Nippon Kayaku Co. Ltd.), jER1032 (manufactured byMitsubishi Chemical Corporation), and the like.

Specific examples of the glycidyl amine-type epoxy resin includetetraglycidyl diaminodiphenyl methanes, glycidyl compounds ofaminophenol, glycidyl anilines, and glycidyl compounds of xylenediamine, and the like.

Commercially available products of tetraglycidyl diaminodiphenylmethanes include Sumiepoxy ELM434 (manufactured by Sumitomo ChemicalCo., Ltd.), Araldite MY720, Araldite MY721, Araldite MY9512, AralditeMY9612, Araldite MY9634, Araldite MY9663 (manufactured by HuntsmanAdvanced Materials), jER604 (manufactured by Mitsubishi ChemicalCorporation), Bakelite EPR494, Bakelite EPR495, “Bakelite EPR496 andBakelite EPR497(manufactured by Bakelite AG), and the like.

Commercially available products of the glycidyl compounds of aminophenoland aminocresol include jER630 (manufactured by Mitsubishi ChemicalCorporation), Araldite MY0500, Araldite MY0510, Araldite MY0600(manufactured by Huntsman Advanced Materials), Sumiepoxy ELM120 andSumiepoxy ELM100 (manufactured by Sumitomo Chemical Co., Ltd.), and thelike.

Commercially available products of glycidyl anilines include GAN, GOT(manufactured by Nippon Kayaku Co., Ltd.), Bakelite EPR493 (manufacturedby Bakelite AG), and the like. The glycidyl compounds of xylene diamineinclude TETRAD-X (manufactured by Mitsubishi Gas Chemical Company Inc.).

Specific examples of the glycidyl ester-type epoxy resins includediglycidyl phthalate ester, diglycidyl hexahydrophthalate ester,diglycidyl isophthalate ester, diglycidyl dimer acid ester, and variousisomers thereof.

Commercially available products of diglycidyl phthalate ester includeEpomic R508 (manufactured by Mitsui Chemicals Inc.), Denacol EX-721(manufactured by Nagase ChemteX Corporation), and the like.

Commercially available products of diglycidyl hexahydrophthalate esterinclude Epomic R540 (manufactured by Mitsui Chemicals Inc.), AK-601(manufactured by Nippon Kayaku Co., Ltd.), and the like.

Commercially available products of diglycidyl dimer acid ester includejER871 (manufactured by Mitsubishi Chemical Corporation), EpotohtoYD-171 (manufactured by Nippon Steel Chemical Co., Ltd.), and the like.

Commercially available products of the alicyclic epoxy resin includeCelloxide 2021P (manufactured by Daicel Corporation), CY179(manufactured by Huntsman Advanced Materials), Celloxide 2081(manufactured by Daicel Corporation), and Celloxide 3000 (manufacturedby Daicel Corporation), and the like.

Epoxy resins having an oxazolidone ring in its skeleton include AER4152,AER4151, LSA4311, LSA4313, LSA7001 (manufactured by Asahi KaseiCorporation), and the like.

Epoxy resins having a naphthalene skeleton in its skeleton includeHP-4032, HP-4700 (manufactured by DIC), NC-7300 (manufactured by NipponKayaku Co., Ltd.), and the like.

Epoxy resins having a dicyclopentadiene skeleton in its skeleton includeXD-100 (manufactured by Nippon Kayaku Co., Ltd.), HP7200 (manufacturedby DIC), and the like.

Epoxy resins having an anthracene skeleton in its skeleton includeYL7172YX-8800 (manufactured by Mitsubishi Chemical Corporation), and thelike.

Epoxy resins having a xanthen skeleton in its skeleton include EXA-7335(manufactured by DIC), and the like.

Preferably, the bisphenol A-type epoxy resin and the epoxy resin havingthe oxazolidone ring in its skeleton are used.

Curing Agent

Amine types, acid anhydride, phenol, mercaptan types, Lewis acid aminecomplex, onium salts, imidazole and the like are used as a curing agentfor the epoxy resin, but those having any structure may be used as longas it can cure the epoxy resin. Preferably, aromatic amine such asdiaminodiphenyl methane and diaminodiphenyl sulfone, imidazolederivatives, dicyandiamide, tetramethylguanidine, thiourea additionamine and isomers and modified ones thereof are included. Dicyandiamideis particularly preferred.

An appropriate curing aid can be combined with these curing agents inorder to enhance a curing activity. As preferred examples, an examplewhere dicyandiamide is combined with a urea compound such as3-phenyl-1,1-dimethylurea, 3-(3,4-dichlorophenyl)-1,1-dimethylurea(DCMU), 3-(3-chloro-4-methylphenyl)-1,1-dimethylurea,2,4-bis(3,3-dimethylureid) toluene, methylenediphenylbis(dimethylureid)or phenyldimethylurea (PDMU) as the curing aid; an example wherecarboxylic anhydride or a novolac resin is combined with tertiary amineas the curing aid; and an example where diaminodiphenyl sulfone iscombined with an imidazole compound, a urea compound such asphenyldimethylurea (PDMU) or an amine complex such as monoethylaminetrifluoride or an amine trichloride complex as the curing aid areavailable. 2,4-bis(3,3-dimethylureid) toluene and methylenephenylbis(dimethylureid) are industrially available as Omicure 24(manufactured by PTI Japan) and Micure 52 (manufactured by PTI Japan),respectively. The preferred curing aid is3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU).

EXAMPLES

Hereinafter, the present invention will be specifically described withreference to Examples. Each measurement method in Examples is asfollows.

1. Composition of Polyacrylonitrile-Based Copolymer

A composition (ratio (mol %) of each monomer unit) of the copolymer wasmeasured as follows using ¹H-NMR method. Dimethylsulfoxide-d6 was usedas a solvent, the copolymer was dissolved therein, and the measurementwas performed using an NMR measurement apparatus (product name: GSZ-400model manufactured by JEOL Ltd.) under a condition of cumulated number:40 and measurement temperature: 120° C., and the ratio of each monomerunit was calculated from an integrated ratio of chemical shift.

2. Specific Viscosity of Polyacrylonitrile-Based Copolymer

A copolymer solution was obtained by dispersing 0.5 g of the copolymerin 100 ml of dimethylformamide and keeping it at 75° C. for 40 minutes.A viscosity η of this solution and a viscosity η0 of the solvent weremeasured at 25° C. using an ubbelohde type viscometer, and a specificviscosity lisp was calculated from the following equation.

ηsp=(η−η₀)/5η₀

3. Melting Point Under Heat and Humidity in Polyacrylonitrile-BasedCopolymer

The copolymer was passed through a sieve with a mesh of 0.5 mm, 5 mgthereof was precisely weighed and placed in a 15 μl sealed sample vesselmanufactured by SII Nanotechnology and made from Ag (for DSC200 system)(treated with heat in air at 300° C. for 30 minutes), and 5 μl of purewater was added and the vessel was sealed tightly. The measurement wasperformed by a heat flux type differential scanning calorimeter at atemperature rising rate of 10° C./minute from room temperature to 230°C. using DSC/220 manufactured by Seiko Instruments Inc., a temperaturecorresponding to a top of an endothermic peak that appeared at around150° C. to 200° C. was read out, and this was used as a melting pointunder heat and humidity Tm (° C.).

4. Contact Angle of Polyacrylonitrile-Based Copolymer With Water

The copolymer was dissolved in dimethylacetamide to prepare a copolymersolution at a mass concentration of 21%, and this copolymer solution wasapplied onto a glass plate so that a constant thickness was obtained.Then, this glass plate on which this copolymer solution had been appliedwas dried in air at 120° C. for 6 hours using a hot wind dryer toevaporate the solvent and make a film having a thickness of 20 to 40 μm.On the surface of this film, 1 μl of water was dropped, a water contactangle was measured 5 times every one second from 3 seconds afterdropping using a contact angle measurement apparatus (product name:DM301 manufactured by Kyowa Interface Science Co., Ltd.,) and their meanvalue θ′ was calculated. Further, a position of the surface on which thewater was dropped was changed, the same manipulation was repeated totalthree times, an arithmetic mean of three times was calculated, and thisvalue was used as the water contact angle θ of the copolymer.

5. Measurement of Oxidation Depth De of Polyacrylonitrile-BasedCopolymer

The copolymer was dissolved in dimethylformamide to prepare a copolymersolution at a mass concentration of 25%, and this copolymer solution wasapplied onto a glass plate so that the constant thickness was obtained.Then, this glass plate on which this copolymer solution had been appliedwas dried in air at 120° C. for 6 hours using the hot wind dryer toevaporate the solvent and make a film having the constant thickness inthe range of 20 to 40 μm. A flame-proof treatment was performed to theobtained film by treating the film with heat in air at 240° C. for 60minutes and further in air at 250° C. for 60 minutes using the hot winddryer. The obtained flame-proofed film was cut into a size oflongitudinal 30 mm and transverse 10 mm, embedded in an epoxy resin, andpolished so that a cross-sectional surface of the film was exposed. Across-section perpendicular to the surface of the polished flame-proofedfilm was observed at a magnification of 1500 using a fluorescencemicroscope (brand name: M1CROFLEX AFX DX). An oxidation progressing partwas observed as a relatively dark layer and an oxidation non-progressingpart was observed as a relatively light layer in the cross-section. Adistance from the surface of the film to a boundary between the darklayer and the light layer was measured at least at 5 points on onecross-section. The same measurement was further repeated on threecross-sections. Their arithmetic mean was used as the oxidation depth De(μm).

6. Single-Fiber Fineness of Precursor Fiber

A single-fiber fineness is a weight per 10000 m of one fiber. Two fiberbundles having a length of 1 m were taken from any site of a precursorfiber bundle, each mass thereof was measured, this value was divided bythe number of filaments (i.e., the number of holes of a nozzle), thenmultiplied by 10000 to calculate a mean value of the two bundles. Thisvalue was used as the single-fiber fineness.

7. Roundness of Precursor Fiber Bundle

(1) Preparation of Sample

A cotton thread is put with half wind on near a center of a lengthwisedirection of a precursor fiber bundle (sample fiber) having a length ofabout 200 mm, and both ends of the cotton thread were combined to passthrough a polyethylene thin tube having a length of about 15 mm (Hibikipolyethylene thin tube No. 3 manufactured by Sansyo Co., Ltd.). At thattime, the sample fiber was attached to an end part of the tube. Then,anti-static agent (Statiside manufactured by Mitsui & Co. Plastics Ltd.)was entirely blown onto the sample fiber (for about 2 seconds). Thecotton thread was drawn to introduce the sample fiber into the tube, andthe tube in which the sample fiber had been placed was cut into about 1to 3 mm on a rubber plate using a razor.

(2) Observation Under SEM

A carbon double stick tape (conductive carbon double stick tape for SEM,width: 8 mm manufactured by Nisshin EM Corporation) was attached on asample stage of SEM, and the sample tube obtained in (1) above wasattached thereon using a precise forceps so that a fiber cross-sectionfaced directly above. Then, the sample was observed using SEM (PHILIPSFEI-XL20), and five photographs on which five or more fibercross-sections were taken were photographed randomly.

(3) Measurement of Roundness

The contour of the fiber cross-section was traced using image analysissoftware (product name: Image-ProPLUS manufactured by Nippon RoperK.K.), then a circumference length L and an area S were measured. Foreach sample, 20 fiber cross-sections from 5 photographs, but 3 or morefrom one photograph were randomly selected and measured, the mean valuesof L and S were obtained, and the roundness was calculated from thefollowing equation. Roundness=(4πS)/L²

8. Constant Velocity Temperature Rising Exothermic Curve of PrecursorFiber Bundle

The constant velocity temperature rising exothermic curve of theprecursor fiber bundle was measured as follows using the heat flux typedifferential scanning calorimeter. First, the precursor fiber bundle wascut into a length of 4.0 mm, 4.0 mg thereof was precisely weighted,packed in a 50 μl sealed sample vessel manufactured by SII and made fromAg (brand name: P/N SSC000E030), and covered with a mesh covermanufactured by SII and made from Cu (brand name: P/N 50-037) (treatedwith heat in air at 450° C. for 15 minutes). Then, the measurement wasperformed using the heat flux type differential scanning calorimeter:DSC/220 manufactured by SII under the condition of a temperature risingrate of 10° C./minute and an air supply amount of 100 ml/minute(standard of the air supply amount: 0.10 MPa at 30° C.) at from roomtemperature (30° C.) to 450° C. A calorific value at 230° C. or aboveand 260° C. or below and a calorific value at 260° C. or above and 290°C. or below in the constant velocity temperature rising exothermic curvewere used as a heat quantity Ja and a heat quantity Jb, respectively.

9. Method of Measuring Calorific Value of Precursor Fiber by Heat FluxType Differential Scanning Calorimeter

The precursor fiber bundle was cut into a length of 2.0 mm, about 7.0 mgthereof was packed in an open sample pan manufactured by SSI and madefrom Al (brand name: P/N SSSC000E030), covered with an SUS mesh cover(brand name: P/N 50-038) (treated with heat in air at 450° C. for 15minutes), and subjected to the measurement of a heat flow. The mass ofthe sample pan, the SUS mesh, and the precursor fiber bundle was weighedusing a precision balance to a 1/100 mg.

The measurement was performed using the heat flux type differentialscanning calorimeter: DSC/220 manufactured by SII as the apparatus underthe condition of a temperature rising rate of 20° C./minute at from roomtemperature to 210° C. and a temperature rising rate of 2° C./minute atfrom 210 to 300° C. and an air supply amount of 100 ml/minute (standardof the air supply amount: 30° C., 0.10 MPa).

A time interval of taking the heat flow is 0.5 seconds. The calorificvalue was obtained by using the heat flow at 215° C. as 0 andintegrating the heat flow at 215 to 300° C. by a time. Specifically, atotal sum of [heat flow (μW)×0.5(s)] at 215 to 300° C. was obtainedusing the temperature and the heat flow at every intake time, and thecalorific value at 215 to 300° C. was obtained. The calorific value wasdivided by a sample amount to obtain the calorific value per unit mass.

10. Method of Measuring Solid ¹H-NMR of Precursor Fiber

The precursor fiber bundle was packed without space into a sample tubemade by cutting a commercially available sample tube for NMR having anexternal diameter of 5 mm into a length of 50 mm so that the lengthwisedirection corresponded with the fiber axis, and subjected to themeasurement. The length of the fiber sample in the sample tube was about6 mm. AVANCEII 300 MHz magnet manufactured by Bruker Bio-Spin was usedas the apparatus. A static probe was used as a probe and set so that thefiber axis was perpendicular to a magnetic field.

When an interval of 90° pulse and 180° pulse in Hahn echo method wasused as τ, a spectrum at τ=6 μS was A and a spectrum at τ=60 μS was B.Then a difference spectrum between A and B was C, and a half-value widthof C was obtained. The difference spectrum can also be obtained byattached analysis software, and the half-value width can also beobtained by the attached analysis software. The measurement condition isas follows.

-   Measurement temperature: 160° C., measurement atmosphere: nitrogen,    Hahn echo method, 90° pulse 5 μs, 180° pulse 10 μs, cumulated    number: 8, and repeat waiting time: 12 s.

11. Diameter and Roundness of Carbon Fiber Bundle

(1) Preparation of Sample

A carbon fiber bundle cut into a length of 5 cm was embedded in an epoxyresin (Epomount base: Epomount curing agent=100:9 (mass ratio)), and cutinto a length of 2 cm to expose a cross-sectional surface, to which amirror surface treatment was given.

(2) Etching Treatment on Surface to be Observed

Further, in order to clarify a contour of the fiber, an etchingtreatment was performed to the cross-sectional surface of the sample bythe following method. Apparatus used: Plasma Etching Apparatus JP-170manufactured by JEOL Ltd. Treatment condition: (Atmospheregas=Ar/O₂=75/25, plasma output power: 50 W, vacuum degree: about 120 Pa,treatment time period: 5 minutes).

(3) Observation Under SEM

The cross-sectional surface of the samples obtained by (1) and (2) abovewas observed using SEM (PHILIPS FEI-XL20), and five photographs of 5 ormore fiber cross-sections on an image were taken randomly.

(4) Measurement of Diameter of Single Fiber in Carbon Fiber Bundle

For each sample, 20 single fiber cross-sections from 5 SEM photographs,but 3 or more from one photograph were randomly selected, the contour ofthe fiber cross-section was traced using image analysis software(product name: Image-ProPLUS manufactured by Nippon Roper K.K.), and amajor axis (maximum feret diameter) d of the cross-section was measured.A mean value of the major axes d of all single fiber cross-sectionsselected was used as the diameter Di of the single fiber in the carbonfiber bundle.

(5) Measurement of Roundness of Single Fiber in Carbon Fiber Bundle

The contour of the fiber cross-section was traced using the imageanalysis software (product name: Image-ProPLUS manufactured by NipponRoper K.K.), and a circumference length L and an area S were measured.For each sample, 20 fiber cross-sections from 5 photographs, but 3 ormore from one photograph were randomly selected and measured, the meanvalues of L and S were obtained, and the roundness was calculated fromthe following equation.

Roundness=(4πS)/L ²

12. Strand Strength and Strand Elastic Modulus of Carbon Fiber

The physical properties (strand strength and strand elastic modulus) ofthe carbon fiber were measured according to the methods described inJISR 7601.

13. Depth of Surface Wrinkle

Several single fibers of the carbon fiber were placed on a sample stage,both ends thereof were secured, and dotite was applied to acircumference thereof to use as a measurement sample. The measurementwas performed using an atom force microscope (product name:SPI3700/SPA-300, manufactured by Seiko Instruments Inc., using acantilever (manufactured by Silicon Nitride) with an AFM mode. Ameasurement image obtained by scanning the single fiber in the range of2 to 7 μm was reversely converted to remove a curvature factor of thefiber after cutting a low frequency components by two dimensionalFourier transform. The depth of the wrinkle was measured five times fromthe cross-section of the plane image thus obtained, and their mean valuewas used as the depth of the surface wrinkle.

14. Evaluation of Fiber Spreading of Carbon Fiber Bundle

A tow width when the carbon fiber bundle was run on a metal roll at arunning speed of 3.4 m/minute under a constant tensile force (0.075cN/dtex) was measured using a digital dimension meter (LS-7030M,manufactured by Keyence), and was used as an indicator of the fiberspreading.

15. Evaluation of Impregnation

The evaluation of the impregnation of the carbon fiber bundle isdescribed using FIG. 3. A carbon fiber bundle 5 was cut out into alength of 30 cm, which was coated with white powder (talc), and one endof the carbon fiber bundle was attached with a clip 7. Formamide 9 ispoured into a container, and a side on which the fiber was attached withthe clip was hung down so that the carbon fiber bundle was perpendicularto a liquid surface. The clip was submerged into formamide, thesubmerging was stopped when the clip was below the liquid surface, thefiber was left to stand for 20 minutes, and the carbon fiber bundle wasimpregnated with formamide. A height at which formamide had impregnatedwas measured after 20 minutes using a ruler 8. This manipulation wasrepeated six times, and their mean value was obtained to use as an“elevated height H”. The higher the elevated height indicates that thebetter the impregnation is. The white powder (talc) was used for makingit easy to confirm the height of the impregnation with formamide. In thecarbon fiber bundle of the present invention, the height of theimpregnation is preferably 100 mm or more.

16. Evaluation of Impregnation in VaRTM Formation

A unidirectional fabric having the weight per unit of 600 g/m² was wovenusing the carbon fiber bundle of the present invention as a warp threadand a line of thread adhering a thermally fused fiber (manufactured byToray Industries Inc.) to a glass fiber of 22.5 tex (manufactured byUnitika Glass fiber Co., Ltd.) as a weft thread and using Repier looms(manufactured by Tsudakoma Corporation). The obtained fabric was cutinto a size of longitudinal 500 mm and transverse 500 mm, and three werelaminated by adjusting to a fiber axis direction. A sheet to be removedafter curing the resin, so-called Peel Ply (nylon taffeta #15) waslaminated on this laminated product (i.e., fiber base material), and amedium (mesh material made from polyethylene, A1RTECH GREENFLOE75) wasplaced thereon for diffusing the resin entirely on the fiber basematerial.

A spiral tube (product No. TSP-10: material polyethylene, thickness: 0.8mm, external diameter: 10 mm, spiral pitch: 11.4 mm manufactured byTrusco) for depositing the resin on both ends of the fiber axisdirection of the fiber base material was disposed, and a suction spoutof a vacuum pump was attached. They were entirely covered with a bagfilm (Lightron #8400), and the circumference of the bag film was adheredto a forming die with a sealing material (Vacuum sealant RS200) so thatthe air did not leak.

Subsequently, this was linked to a discharge spout of the resin injectedfrom a resin tank. As the resin, an epoxy resin for molding infusion(main base: DENATITE XNR6815 and curing agent: DENATITE XNH6815manufactured by Nagase ChemteX Corporation) was combined in a ratio of100 parts by mass of the main base and 27 parts by mass of the curingagent (viscosity of the mixture 260 mPa·S) for the use. Then, the fiberbase material covered with the bag film was vacuumized to a vacuumpressure of about 70 to 76 cmHg using the vacuum pump, and then thevalve was opened and the resin was injected.

At that time, the time until the impregnation with the resin wascompleted was measured to evaluate the impregnation with the resin. Theimpregnation with the resin was evaluated by using the time fromstarting the injection of the resin until the three fabrics wereentirely impregnated with the resin as a determination criterion. Theimpregnation was evaluated as follows.

-   A: Impregnation time was less than 10 minutes-   B: Impregnation time was 10 minutes or more.

Example 1

To a 80 liter aluminium polymerization kettle equipped with a turbineagitator wing (agitator wing: 240ϕ, 4 wings of two steps of 55 mm×57mm), 76.5 L of deionized exchange water was added to reach an over flowspout of the kettle, 0.01 g of ferrous sulfate (Fe₂SO₄.7H₂O) was added,pH of the reaction was adjusted to 3.0 using sulfuric acid, and thetemperature in the polymerization kettle was kept at 57° C.

Subsequently, ammonium persulfate at 0.10 mol %, ammonium hydrogensulfide at 0.35 mol %, ferrous sulfate (Fe₂SO₄.7H₂O) at 0.3 ppm andsulfuric acid at 5.0×10⁻² mol %, which were redox polymerizationinitiators for the monomer were dissolved in deionized exchange water,respectively. These solution were continuously supplied from 50 minutesbefore starting the polymerization, agitation was performed at anagitation speed of 180 rpm and an agitation power of 1.2 kW/m³, and theaverage retention time of the monomer in the polymerization kettle wasset to be 70 minutes.

Then, the monomer composed of acrylonitrile (hereinafter abbreviatedas“AN”) at 98.7 mol % and 2-hydroxyethyl methacrylate (hereinafterabbreviated as“HEMA”) at 1.3 mol % was constituted to makewater/monomer=3 (mass ratio) at the start of the polymerization, andthis polymer solution was continuously supplied. Subsequently, one hourafter starting the polymerization, the polymerization reactiontemperature was lowered to 50° C., this temperature was kept, andpolymerization slurry was continuously taken out from the overflow spoutof the polymerization kettle.

An aqueous solution of a polymerization terminator obtained bydissolving sodium oxalate at 0.37×10⁻² mol % and sodium bicarbonate at1.78×10⁻² mol % in deionized exchange water was added to thepolymerization slurry to make pH of the polymerization slurry 5.5 to6.0. This polymerization slurry was dehydrated by an Oliver-typecontinuous filter, and deionized exchange water (70 L) in an amount thatwas 10 times larger than that of the polymer was added to disperseagain. The polymer slurry after being dispersed again was dehydratedagain by the Oliver-type continuous filter, a pellet was formed anddried using a hot wind circulating dryer at 80° C. for 8 hours, andpulverized in a hammer mill to obtain a polyacrylonitrile-basedcopolymer A. In the obtained copolymer A, its composition was composedof 98.5 mol % of an AN unit and 1.5 mol % of an HEMA unit, a specificviscosity was 0.21, and a melting point under heat and humidity was 170°C. Further, a water contact angle and an oxidation depth De in thiscopolymer A were 62.3° and 4.5 μm, respectively.

This copolymer was dissolved in an organic solvent such asdimethylacetamide to prepare a spinning neat solution at a concentrationof 21% by mass. Then, threads were spun by a wet spinning method under acoagulation bath condition of a concentration of 60% by mass andtemperature of 35° C. to obtain a precursor fiber bundle. A single-fiberfineness, the filament number and a fiber density in this precursorfiber bundle were 2.0 dtex, 30000 and 1.18 g/cm³, respectively, and itscross-sectional shape had a roundness of 0.85 and a horsebean shape.Further, heat quantities Ja and Jb obtained by the measurement using theheat flux type differential scanning calorimeter were 185 kJ/kg and 740kJ/kg, respectively.

A flame-proof treatment in air heated at 250 to 290° C. in a hot windcirculating flame-proof furnace with an extension rate of +2% for 60minutes was performed to this precursor fiber bundle to obtain aflame-proofed fiber bundle. A density of the obtained flame-proofedfiber bundle was 1.392 g/cm³.

Subsequently, a carbonization treatment was performed to thisflame-proofed fiber bundle by treating with low temperature at thehighest temperature of 660° C. with an extension rate of 3.0% for 1.5minutes under a nitrogen atmosphere and treating in a high temperaturetreatment furnace at the highest temperature of 1350° C. with anextension rate of −4.5% for 1.5 minutes under a nitrogen atmosphere toobtain a carbon fiber bundle.

The diameter Di and the roundness of the obtained carbon fiber were 9.43μm and 0.84, respectively. Further, a strand tensile strength was 4300MPa and a strand tensile elastic modulus was 245 GPa, which were high.This is because the compactness and homogeneity enough for performancereappearance of the carbon fiber are kept by containing the HEMA unit inthe precursor fiber, an exothermic property to sufficiently diffuse theoxygen inside the fiber is kept even when the flame-proof treatment athigh temperature for a short period of time, and additionally the evenflame-proof treatment becomes possible by the short distance from thesurface to the center of the cross-section of the fiber due to thehorsebean shape of the fiber cross-section in the precursor fiber.

Examples 2 to 15

Copolymers A, B, C or F, G were obtained in the same manner as inExample 1, except that a supply ratio (molar ratio) of the monomers uponstart of the polymerization was changed as shown in Table 1 or Table 2.HPMA and HEA in Table 1 or 2 are 2-hydroxypropyl methacrylate and2-hydroxyethyl acrylate, respectively. The composition, the specificviscosity, the melting point under heat and humidity of the obtainedcopolymers, and the water contact angle and the oxidation depth De ofthe film obtained from each copolymer are shown in Table 1 or Table 2.

A spinning neat solution was prepared using each of these copolymers andthe threads were spun in the same manner as in Example 1 to obtain aprecursor fiber bundle. The single fiber fineness, the filament number,the fiber density, the coagulation bath condition, the roundness, thecross-sectional shape, the heat quantities Ja and Jb of each precursorfiber bundle are shown in Table 1 or Table 2.

Then, the flame-proof treatment was given to each of these precursorfiber bundles in the hot wind circulating flame-proof furnace in heatedair at temperature with the extension rate for the time shown in Table 1or Table 2. The density of obtained each flame-proofed fiber is shown inTable 1 or Table 2.

Further, the carbonization treatment was performed to this flame-proofedfiber bundle in the same manner as in Example 1 to obtain a carbon fiberbundle. The diameter, the roundness, the fiber spreading, the strandtensile strength, and the strand elastic modulus of the obtained carbonfiber are shown in Table 1 or Table 2.

The cross-sectional shape of the carbon fiber obtained in Examples 2 to15 is the horsebean type having the roundness of 0.78 to 0.88, and boththe strand tensile strength and the strand tensile elastic modulusexhibited the high values. This is because the precursor fiber has thesufficient compactness and homogeneity and the even flame-prooftreatment is possible like Example 1. It was also confirmed that the towwidth in the obtained carbon fiber bundle was wider than that obtainedfrom the precursor fiber bundle having a round cross-sectional shape andhaving the same single fiber fineness, and the obtained carbon fiberbundle was excellent in fiber spreading.

Comparative Examples 1 to 14

Copolymers A, B, D or E were obtained in the same manner as in Example1, except that a supply ratio (molar ratio) of the monomers upon startof the polymerization was changed as shown in Table 3 or Table 4. AAm,MAA, and IBMA in Table 3 or Table 4 are acrylamide, methacrylic acid andisobutyl methacrylate, respectively. The composition, the specificviscosity, the melting point under heat and humidity of the obtainedcopolymers, and the water contact angle and the oxidation depth De ofthe film obtained from each copolymer are shown in Table 3 or Table 4.

A spinning neat solution was prepared using each of these copolymers andthe threads were spun in the same manner as in Example 1 to obtain aprecursor fiber bundle. The single fiber fineness, the filament number,the fiber density, the coagulation bath condition, the roundness, thecross-sectional shape, the heat quantities Ja and Jb of each precursorfiber bundle are shown in Table 3 or Table 4.

Then, the flame-proof treatment was performed to each of these precursorfiber bundles in the hot wind circulating flame-proof furnace in heatedair at temperature with the extension rate for the time shown in Table 3or Table 4. The density of obtained each flame-proofed fiber is shown inTable 3 or Table 4.

Further, the carbonization treatment was given to this flame-proofedfiber bundle in the same manner as in Example 1 to obtain a carbon fiberbundle. The diameter, the roundness, the fiber spreading, the strandtensile strength, and the strand elastic modulus of the obtained carbonfiber are shown in Table 3 or Table 4.

The cross-sectional shape of the carbon fiber obtained in ComparativeExample 1 was a round type having the diameter of 7.6 μm and theroundness of 0.95. Further, the strand tensile strength and the strandtensile elastic modulus exhibited low values, which were 1910 MPa and222 GPa, respectively. This is because the distance from the surface tothe center of the cross-section of the fiber was long due to the roundtype of the fiber cross-sectional shape in the precursor fiber and thusthe flame-proof treatment could not be given evenly.

The cross-sectional shape of the carbon fiber obtained in ComparativeExample 2 was the horsebean type having the diameter of 9.4 μm and theroundness of 0.85. However, the strand tensile strength and the strandtensile elastic modulus exhibited lower values than in Example 1. Thisis because the compactness or the homogeneity of the precursor fiberbundle could not be kept due to the monomer such as the HEMA unit havingthe hydrophilic group was not contained in the copolymer that composesthe precursor fiber and thus the water contact angle of the film was74.4° that was very high, and the precursor fiber was plasticized beforethe flame-proof reaction progressed and the fiber was extended in theflame-proof step because the heat quantity Ja was 52 kJ/kg that was verysmall. In the condition of this Comparative Example, the heat quantityJa was 340 kJ/kg that was very small. Thus, a flame-proof reactivity islow, a very long time is required for the flame-proof treatment, and theproductivity is prominently impaired.

The cross-sectional shape of the carbon fiber obtained in ComparativeExample 3 was the horsebean type having the diameter of 9.4 μm and theroundness of 0.81. However, the strand tensile strength and the strandtensile elastic modulus exhibited lower values than in Example 1. Thisis because, since a site of the carboxyl group in the copolymer thatcomposes the precursor fiber was not hydroxyalkylated, the heat quantityJb was 1150 kJ/kg that was very high, the flame-proof reactionprogressed at once, and the cross-section double structure was easilyformed. Also, because the oxidation depth De of the film was 3.0 μm thatwas small and the oxygen permeability was low, which meant that theoxygen could not be diffused inside the precursor fiber having the largesingle fiber fineness and the flame-proof treatment could not be givenevenly.

The cross-sectional shape of the carbon fiber obtained in ComparativeExample 4 was the horsebean type having the diameter of 11.9 μm and theroundness of 0.82. However, the strand tensile strength and the strandtensile elastic modulus exhibited the lower values than in Example 1.This seems to be due to the same reasons as in Comparative Example 3.

In Comparative Example 5, the carbon fiber bundle could not be sampled.This is because, since the site of the carboxyl group in the copolymerthat composes the precursor fiber was not hydroxyalkylated, the heatquantity Jb was 1150 kJ/kg that was very high, the flame-proof reactionprogressed at once, and the cross-section double structure was easilyformed. Also, because the oxidation depth D of the film was 3.0 μm thatwas small and the oxygen permeability was low, which meant that theoxygen could not be diffused inside the precursor fiber having the largesingle-fiber fineness, the flame-proof treatment could not be givenevenly and the formation of the cross-section double structure wasprominent.

The cross-sectional shape of the carbon fiber obtained in ComparativeExample 6 was the horsebean type having the diameter of 11.7 μm and theroundness of 0.82. However, the strand tensile strength and the strandtensile elastic modulus exhibited lower values than in Example 1. Thisis because, since the monomer unit such as the HEMA unit having thehydrophilic group was not contained in the copolymer that composes theprecursor fiber, the water contact angle of the film was 76.2° that wasvery high and the compactness or the homogeneity of the precursor fiberbundle could not be kept. Further, because the copolymer H contains theIA unit, the heat quantity Ja is 178 kJ/kg that is large whereas theheat quantity Jb is 473 kJ/kg that is very small, and therefore theflame-proof treatment cannot be given evenly even when the treatmenttime is prolonged to 100 minutes.

The cross-sectional shape of the carbon fiber obtained in ComparativeExample 7 was the horsebean type having the diameter of 12.3 μm and theroundness of 0.81. However, the strand tensile strength and the strandtensile elastic modulus exhibited lower values than in Example 1. Thisis because, since the monomer unit such as the HEMA unit having thehydrophilic group was not contained in the copolymer that composes theprecursor fiber, the water contact angle of the film was 71.1° that washigh and the compactness or the homogeneity of the precursor fiberbundle could not be kept. Further, because the copolymer 1 contains anMAA unit, the heat quantity Ja is 262 kJ/kg that is very high, theflame-proof reaction progresses at once, and thus the cross-sectiondouble structure is easily formed. Also, because the oxidation depth Deof the film is 3.2 μm that is small, the oxygen permeability of theprecursor fiber is low, which meant that the oxygen could not bediffused inside the precursor fiber having the large single fiberfineness and the flame-proof treatment could not be given evenly.

The cross-sectional shape of the carbon fiber obtained in ComparativeExample 8 was the horsebean type having the diameter of 11.9 μm and theroundness of 0.83. However, the strand tensile strength and the strandtensile elastic modulus exhibited lower values than in Example 1. Thefollowings seem to be causes for this. The compactness and thehomogeneity of the precursor fiber are kept when the AAm unit iscontained in the copolymer that composes the precursor fiber. But, sincethe site of the carboxyl group in the copolymer is not hydroxyalkylated,the heat quantity Ja is 82 kJ/kg that is very small, the precursor fiberis plasticized before the flame-proof reaction progresses, and the fiberis extended in the flame-proof step. In addition, the cross-sectiondouble structure is easily formed because the heat quantity Jb is 1098kJ/kg that is high and the flame-proof reaction progresses at once.Also, because the monomer unit including the carboxylate group is notcontained in the copolymer J whereas the bulky IBMA unit is introducedas the monomer unit, the oxidation depth De of the film is 6.3 μm thatis large, the oxygen permeability of the precursor fiber is enough butthe flame-proof reactivity is unsuitable, and therefore the flame-prooftreatment cannot be given evenly.

The cross-sectional shape of the carbon fiber obtained in ComparativeExample 9 was the horsebean type having the diameter of 7.1 μm and theroundness of 0.84. Further, the strand tensile strength and the strandtensile elastic modulus exhibited the similar values to those in Example1, but the tow width that was the indicator for the fiber spreading was20.9 mm that was the lowest in the all Examples of the presentinvention. This is because the single fiber fineness of the precursorfiber bundle is 1.0 dtex that is thin, and thus the single fibers areeasily tangled with one another and the fiber spreading is reduced.

The strand tensile strength and the strand tensile elastic modulus ofthe carbon fibers obtained in Comparative Examples 10 to 14 are lowerthan those in Example 1. This is because the fiber cross-sectional shapeof the precursor fiber is the round type, where the distance from thesurface to the center of the cross-section of the fiber is long, andthus the flame-proof treatment cannot be given evenly.

Example 16

An acryl-based copolymer A containing 98.0 mol % of the AN unit and 2.0mol % of the HEMA unit and having the specific viscosity of 0.21produced in the same manner as in Example 1 was dissolved indimethylacetamide to prepare a spinning neat solution where the polymerconcentration was 21% and a neat solution temperature was 60° C. Threadswere spun by the wet spinning method using this spinning neat solution.A coagulation bath into which the spinning neat solution is dischargedis an aqueous solution of dimethylacetamide at a concentration of 45% bymass at temperature of 25° C. The number of holes in a spinning nozzleused is 3000. Coagulated threads obtained by coagulating in thecoagulation bath were stretched with washing and stretched with heat tototal 7.4 times to obtain a precursor fiber bundle A. A discharge amountwas controlled so that the single-fiber fineness of the precursor fiberbundle A was 2.5 dtex. The calorific value obtained by the heat fluxtype differential scanning calorimeter was 3400 kJ/kg and the H-NMRhalf-value width was 12.5 kHz.

The flame-proof treatment was given to the precursor fiber bundle A bytreating it in air heated at 230 to 270° C. at an extending rate of 2%for 70 minutes in the hot wind circulating flame-proof furnace to obtaina flame-proofed fiber bundle. The temperature in the flame-proof furnacewas controlled so that the density of the flame-proofed fiber becameabout 1.35 g/cm³ in 70 minutes. The density of the obtainedflame-proofed fiber was 1.352 g/cm³.

Subsequently, this flame-proofed fiber bundle was treated with heat atthe highest temperature of 690° C. at an extending rate of 3.0% for oneminute under a nitrogen atmosphere (pre-carbonization treatment), andfurther carbonized in a high temperature treatment furnace at thehighest temperature of 1450° C. at an extending rate of −4.3% for oneminute under a nitrogen atmosphere to obtain a carbon fiber bundle. Thestrand tensile strength and the strand tensile elastic modulus were 4390MPa and 251 GPa, respectively that were high.

Example 17

The flame-proof treatment was given to the precursor fiber bundle Aproduced in the same manner as in Example 16 by treating it in the airheated at 230 to 270° C. at an extending rate of 2% for 90 minutes inthe hot wind circulating flame-proof furnace. The temperature in theflame-proof furnace was controlled so that 1.40 g/cm³ was obtained in 90minutes. The density of the obtained flame-proofed fiber was 1.400.

Subsequently, the pre-carbonization treatment and the carbonizationtreatment were given in the same manner as in Example 16 to obtain acarbon fiber bundle. The strand tensile strength and the strand tensileelastic modulus were 4280 MPa and 260 GPa, respectively, that were high.

Examples 18 to 27 and Comparative Examples 15 to 19

A spinning neat solution was prepared and threads were spun to obtainprecursor fiber bundles B to I in the same manner as in Example 16except that the coagulation concentration and the coagulationtemperature were values shown in Table 5 and the discharge amount wascontrolled so that the single-fiber fineness was the value shown inTable 5.

The single-fiber fineness, the calorific value obtained by the heat fluxtype differential scanning calorimeter, and the ¹H-NMR half-value widthin the obtained precursor fiber bundles are shown in Table 6.

Then, the flame-proof treatment was given to each of these precursorfiber bundles in the hot wind circulating flame-proof furnace under thecondition in Example 16 or Example 17. The density of the obtained eachflame-proofed fiber is shown in Table 6.

Further, the pre-carbonization treatment and the carbonization treatmentwere given to this flame-proofed fiber bundle in the same manner as inExample 16 or Example 17 to obtain a carbon fiber bundle. The strandtensile strength and the strand tensile elastic modulus of the obtainedcarbon fibers are shown in Table 6.

The carbon fibers obtained in Examples 18 to 27 exhibited the highvalues of the strand tensile strength and the strand tensile elasticmodulus.

In Comparative Examples 15 and 16, the calorific value per unit mass issmaller than 3200 kJ/kg and thus the strand elastic modulus is smallerthan that in Examples. In Comparative Examples 17 and 18, both thestrand tensile strength and the strand tensile elastic modulus are good,but the single-fiber fineness is 1.5 dtex that is small. Thus, theobjective carbon fiber bundle could not be obtained.

Example 28

A precursor fiber bundle J was obtained in the same manner as in Example16, except that the copolymer B produced in the same manner as inExample 15, containing 98.5 mol % of the AN unit and 1.5 mol % of theHEMA unit and having the specific viscosity of 0.21 was used, thedischarge amount was controlled so that the single-fiber fineness was2.0 dtex, and the coagulation bath condition in Table 5 was used.

The flame-proof treatment was given to the precursor fiber bundle J bytreating it in the heated air at 230 to 270° C. at an extending rate of2% for 60 minutes in the hot wind circulating furnace to obtain aflame-proofed fiber bundle. The temperature in the flame-proof furnacewas controlled so that the density of the flame-proofed fiber becameabout 1.35 g/cm³ after treating for 60 minutes. After the flame-prooftreatment, the pre-carbonization treatment and the subsequentcarbonization treatment were performed under the same condition as inExample 16 to obtain a carbon fiber bundle. Evaluation results are shownin Table 6.

Comparative Examples 20 and 21

An acryl-based copolymer D containing 97.0 mol % of the AN unit, 2.6 mol% of the AAm unit and 0.4 mol % of a methacrylic acid unit and havingthe specific viscosity of 0.21 was used, a spinning neat solution wasprepared and the discharge amount was controlled in the same manner asin Example 16, and the threads were spun under the coagulation bathcondition shown in Table 5 to obtain precursor fiber bundles K and L.The single fiber fineness, the calorific value obtained by the heat fluxtype differential scanning calorimeter, and the ¹H-NMR half-value widthin the obtained precursor fiber bundles are shown in Table 6. And theflame-proof treatment was given to this flame-proofed fiber bundle bycontrolling the flame-proof temperature so that the density of theflame-proofed fiber became about 1.35 g/cm³ in 70 minutes, andsubsequently the pre-carbonization treatment and the carbonizationtreatment were performed under the same condition as in Example 16 toobtain a carbon fiber bundle. Evaluation results are shown in Table 6.

In Comparative Example 20, the calorific value per unit mass is smallerthan 3200 kJ/kg and the ¹H-NMR half-value width is larger than 14.5 kHz.Thus the oxygen is not diffused sufficiently and there are few stablestructures. Thus, both the strand tensile strength and the strandtensile elastic modulus were low.

In Comparative Example 21, the ¹H-NMR half-value width is larger than14.5 kHz. Thus, a diffusion rate of the oxygen is slow. However, thequantity of change to the stable structure upon the flame-prooftreatment becomes large because the single fiber fineness is 1.5 dtexthat is thin. Thus, the strand tensile strength and the strand tensileelastic modulus of the carbon fiber exhibited the high values.Meanwhile, the single fiber fineness is small. Thus the carbon fiberhaving the target thickness of the single fiber could not be obtained.

A relation between the strand elastic modulus and the calorific valueper unit mass in Examples and Comparative Examples is shown in FIG. 6.It is evident that when the calorific value per unit mass is smallerthan 3200 kJ/kg, the strand tensile elastic modulus is reduced and thephysical property is reduced regardless of the copolymer composition andthe single fiber fineness.

Example 29

Acrylonitrile and 2-hydroxyethyl methacrylate were copolymerized byaqueous suspension polymerization in the presence of ammoniumpersulfate/ammonium hydrogen sulfide and iron sulfate to obtain anacrylonitrile-based copolymer composed of acrylonitrileunit/2-hydroxyethyl methacrylate unit=98.5/1.5 (mol %). This copolymerwas dissolved in dimethylacetamide to prepare a spinning neat solutionat 21% by mass. The spinning neat solution was discharged into acoagulation bath composed of an aqueous solution of dimethylacetamide ata concentration of 60% by mass at temperature of 35° C. through aspinning nozzle having a hole number of 24,000 and a hole diameter of 60μm, and a fiber bundle (swelled line of threads) was obtained byreceiving at a velocity 0.32 times a discharge linear velocity from thesurface of the spinning nozzle.

Then, this fiber bundle was washed with water simultaneously withstretching it to 5.4 times, further led to a first oil bath composed ofan oil treatment solution in which an oil composition of amino-modifiedsilicon/polyoxyethylene (6) lauryl ether=91/9 (mass ratio) was dispersedat a concentration of 1.5% by mass in water, and the oil treatmentsolution was imparted to the fiber bundle. Then, the oil treatmentsolution was once squeezed out at a guide, subsequently, the fiberbundle was led to a second oil bath composed of the same composition andconcentration as in the first oil bath, and the oil treatment solutionwas imparted again to the fiber bundle.

The fiber bundle to which the oil treatment solution had been impartedagain was dried using a heating roll, and stretched with dry heat to1.34 times between the heating rolls in which a rotation speed wasadjusted to a predetermined condition. A total stretching magnificationfrom the swelled line of threads was 7.4 times. Subsequently, a waterpercentage is controlled by giving water to the fiber bundle at a touchroll to obtain a precursor fiber bundle having the single fiber finenessof 2.5 dtex.

The flame-proof treatment was given to this precursor fiber bundle bytreating it in air heated at 220 to 260° C. at an extending rate of−1.5% for 70 minutes in the hot wind circulating flame-proof furnace toobtain a flame-proofed fiber bundle. The obtained flame-proofed fiberbundle was further pre-carbonized under the nitrogen atmosphere at 700°C. at an extending rate of +3% for 1.4 minutes, and subsequentlycarbonized under the nitrogen atmosphere at 1,300° C. at an extendingrate of −4.0% for 1.0 minute to obtain a carbon fiber. Subsequently,after giving a surface treatment, a sizing agent obtained by mixing 80parts by mass of “Epicoat 828” as a main base manufactured by JapanEpoxy Resin and 20 parts by mass of “Pluronic F88” as an emulsifiermanufactured by Asahi Denka and preparing a water dispersion solution byphase transfer emulsification was adhered at 1% by mass thereto, and adrying treatment was given thereto to obtain a carbon fiber bundle.

The single fiber fineness of the obtained carbon fiber bundle was 1.3dtex. The strand strength and the strand elastic modulus were 4300 MPaand 233 GPa, respectively. The roundness of the cross-section and thedepth of the wrinkle in this carbon fiber were 0.75 and 49.8 nm,respectively. The impregnation was evaluated, and the elevation heightwas 126 mm. The VaRTM processing was performed and the impregnation withthe resin was evaluated, and the impregnation time was 9 minutes and theimpregnation was good. The evaluation results are summarized in Table 7.

Example 30

A carbon fiber bundle was obtained in the same manner as in Example 29,except that acrylonitrile and 2-hydroxyethyl methacrylate werecopolymerized by aqueous suspension polymerization in the presence ofammonium persulfate/ammonium hydrogen sulfide and iron sulfate to obtainan acrylonitrile-based copolymer composed of acrylonitrileunit/2-hydroxyethyl methacrylate unit=98.0/2.0 (mol %).

The single fiber fineness of the obtained carbon fiber bundle was 1.3dtex. Strand physical properties were measured. The strand strength andthe strand elastic modulus were 4.2 GPa and 232 GPa, respectively. Theroundness of the cross-section and the depth of the wrinkle in thiscarbon fiber were 0.75 and 50.0 nm, respectively. The impregnation wasevaluated, and the elevation height was 125 mm. The VaRTM processing wasperformed and the impregnation with the resin was evaluated, and theimpregnation time was 9 minutes and the impregnation was good.

Comparative Example 22

A precursor fiber bundle having the single fiber fineness of 4.5 dtexwas obtained in the same manner as in Example 29, except the spinningcondition shown in Table 7.

The flame-proof treatment was given to this precursor fiber bundle bytreating it in air heated at 220 to 260° C. at an extending rate of−5.9% for 70 minutes in the hot wind circulating flame-proof furnace toobtain a flame-proofed fiber bundle. It was tried to furtherpre-carbonize the obtained flame-proofed fiber bundle under the nitrogenatmosphere at 700° C. at an extending rate of +3%, but many brokenthreads were produced in the pre-carbonization step probably due toshortage of the flame-proof treatment. Thus, the flame-proof treatmentwas performed at an extending rate of −5.9% for 300 minutes, and thefiber bundle could pass through the pre-carbonization step without thebroken thread. Subsequently, the fiber bundle was carbonized under thenitrogen atmosphere at 1,300° C. at an extending rate of −4.0% for 3.2minutes to obtain a carbon fiber bundle. A carbon fiber bundle wasobtained in the same manner as in Example 29, except for aboveconditions. Each evaluation result is shown in Table 7.

Comparative Example 23

Acrylonitrile, acrylamide and methacrylic acid were copolymerized byaqueous suspension polymerization in the presence of ammoniumpersulfate/ammonium hydrogen sulfide and iron sulfate to obtain anacrylonitrile-based copolymer composed of acrylonitrile unit/acrylamideunit/methacrylic acid unit=96/3/1 (mol %). Using this copolymer, thepreparation of a spinning neat solution, spinning, washing with water,stretching, and the treatment with the oil agent were performed in thesame manner as in Example 29 to impart the oil treatment solution to thefiber bundle.

The fiber bundle to which the oil treatment solution had been impartedagain was dried using the heating roll, and stretched with dry heat to1.3 times between the heating rolls in which the rotation speed wasadjusted to the predetermined condition. A total stretchingmagnification from the swelled line of threads was 7.3 times.Subsequently, the water percentage is controlled by giving water to thefiber bundle at the touch roll to obtain a precursor fiber bundle havingthe single fiber fineness of 2.5 dtex.

The flame-proof treatment was given to the above precursor fiber bundleby treating it in air heated at 220 to 260° C. at an extending rate of−5.9% for 70 minutes in the hot wind circulating flame-proof furnace toobtain a flame-proofed fiber bundle. It was tried to furtherpre-carbonize the obtained flame-proofed fiber bundle under the nitrogenatmosphere at 700° C. at an extending rate of +3%, but many brokenthreads were produced in the pre-carbonization step probably due toshortage of the flame-proof treatment. Thus, the flame-proof treatmentwas performed at an extending rate of −5.9% for 300 minutes, and thefiber bundle could pass through the pre-carbonization step without thebroken thread. Subsequently, the fiber bundle was carbonized under thenitrogen atmosphere at 1,300° C. at an extending rate of −4.0% for 3.2minutes to obtain a carbon fiber bundle. A carbon fiber bundle wasobtained in the same manner as in Example 29, except for aboveconditions, and the evaluation results in Table 7 were obtained.

Comparative Example 24

A PAN-based precursor fiber bundle having the single fiber fineness of1.0 dtex was obtained and further the carbon fiber was produced in thesame manner as in Example 29, except for the spinning condition shown inTable 7, and the evaluation results in Table 7 were obtained.

Comparative Example 25

A fiber bundle (swelled line of threads) was obtained by employing thespinning condition shown in Table 7. Then, this fiber bundle was washedwith water simultaneously with stretching to 4.8 times, and the oiltreatment solution was imparted to the fiber bundle in the same manneras in Example 29. This fiber bundle was dried using the heating roll,and stretched to 2.7 times with steam using a steam stretching machine.At this time, the total stretching magnification from the swelled lineof threads was 12.7 times. A precursor fiber bundle having the singlefiber fineness of 1.2 dtex was obtained in the same manner as in Example29, except for above conditions.

The flame-proof treatment was given to this precursor fiber bundle bytreating it in air heated at 220 to 260° C. at an extending rate of−6.0% for 60 minutes in the hot wind circulating flame-proof furnace toobtain a flame-proofed fiber bundle. The obtained flame-proofed fiberbundle was pre-carbonized under the nitrogen atmosphere at 700° C. at anextending rate of +3% for 1.6 minutes, and subsequently carbonized underthe nitrogen atmosphere at 1,250° C. at an extending rate of −4.6% for1.4 minutes to obtain a carbon fiber. A carbon fiber bundle was obtainedin the same manner as in Example 29, except for above conditions. And,the evaluation results in Table 7 were obtained.

Comparative Example 26

A fiber bundle (swelled line of threads) was obtained by employing thespinning condition shown in Table 7. Then, this fiber bundle was washedwith water simultaneously with stretching to 5.9 times, and the oiltreatment solution was imparted to the fiber bundle in the same manneras in Example 29. This fiber bundle was dried using the heating roll,and stretched to 2.1 times with steam using the steam stretchingmachine. At this time, the total stretching magnification from theswelled line of threads was 12.5 times. A precursor fiber bundle havingthe single fiber fineness of 1.2 dtex was obtained in the sme manner asin Example 29, except for above conditions.

A carbon fiber was produced using this precursor fiber bundle in thesame manner as in Comparative Example 23, and the evaluation results inTable 7 were obtained.

Comparative Example 27

A precursor fiber bundle having the single fiber fineness of 2.5 dtexwas obtained in the same manner as in Example 29, except for thespinning condition shown in Table 7. It was tried to produce a carbonfiber using this precursor fiber bundle in the same manner as in Example29, but the bundle integrity of the fiber bundle was reduced, thepassing property through the burning step when the carbon fiber bundlewas produced was deteriorated, and the carbon fiber bundle could not beproduced stably.

Examples 31 and 32 and Comparative Examples 28 to 30 1. Raw Materials

The followings were used as raw materials in the following Examples andComparative Examples.

(1-1. Carbon Fiber)

PAN-based carbon fiber 1 (single fiber fineness: 0.75 dtex, roundness:0.70, diameter Di: 8.4 μm, strength: 4116 MPa, elastic modulus: 235 GPa)

PAN-based carbon fiber 2 (single fiber fineness: 1.24 dtex, roundness:0.75, diameter Di: 11.9 μm, strength: 4226 MPa, elastic modulus: 229GPa)

PAN-based carbon fiber 3 (single fiber fineness: 2.01 dtex, roundness:0.73, diameter Di: 15.6 μm, strength: 3489 MPa, elastic modulus: 246GPa)

PAN-based carbon fiber 4 (single fiber fineness: 1.21 dtex, roundness:0.95, diameter Di: 9.6 μm, strength: 3989 MPa, elastic modulus: 227 GPa)

PAN-based carbon fiber 5 (single fiber fineness: 2.29 dtex, roundness:0.95, diameter Di: 11.9 μm, strength: 3283 MPa, elastic modulus: 232GPa)

The PAN-based carbon fiber 1 was produced under the same condition as inComparative Example 1, except that the number of filaments was changedto 50000. The PAN-based carbon fiber 2 was produced under the samecondition as in Example 3. The PAN-based carbon fiber 3 was producedunder the same condition as in Example 15, except that the number offilaments was changed to 12000. The PAN-based carbon fiber 4 wasproduced under the same condition as in Comparative Example 12. ThePAN-based carbon fiber 5 was produced under the same condition as inComparative Example 14, except that the number of filaments was changedto 12000 and the fineness of the carbon fiber precursor was changed to4.5 dtex.

(1-2. Epoxy Resin)

jER828: Liquid bisphenol A-type epoxy resin (manufactured by MitsubishiChemical Corporation)

AER4152: Oxazolidone-type epoxy resin (manufactured by Asahi KaseiCorporation)

(1-3. Thermoplastic Resin)

Vinylec E: polyvinyl formal resin (manufactured by Chisso Corporation)

(1-4. Curing Aid)

DCMU: urea compound DCMU 99 (manufactured by Hodogaya Chemical Co.,Ltd.)

PDMU: urea compound Omicure 94 (manufactured by PTI Japan).

(1-5. Curing Agent)

DICY: dicyandiamide DICY 15 (manufactured by Mitsubishi ChemicalCorporation)

2. Production and Evaluation

The following production condition and evaluation condition wereemployed in the following Examples and Comparative Examples.

(2-1. Preparation of Epoxy Resin Composition)

The epoxy resin and the thermoplastic resin in predetermined amountswere added in a kneader, kneaded while the temperature was raised up to160° C., and kneaded at 160° C. for one hour to obtain a clear viscoussolution. The temperature was lowered down to 60° C. while the solutionis kneaded, and the curing aid and the curing agent in predeterminedamounts were added and kneaded to obtain an epoxy resin composition. Thecomposition of this epoxy resin composition is shown in Table 8.

(2-2. Measurement of Flow Index)

The flow index was measured by the method described previously.

(2-3. Preparation of Resin Film)

The epoxy resin composition obtained by the above preparation of theepoxy resin composition was applied onto mold-releasing paper at a resinweight per unit of 50 to 55 g/m² at 60° C. using a film coater to obtaina “resin film T”.

(2-4. Preparation of Carbon Fiber Prepreg)

A unidirectional prepreg in which the weight per unit of the carbonfiber was 202 to 213 g/m² and a resin content rate was 32.0 to 34.3% bymass was obtained by twisting each carbon fiber bundle (any of thePAN-based carbon fiber bubbles 1 to 5) on the resin-applied surface ofthe resin film T using a drum wind, placing another resin film T thereonso that the applied surface faced downward to sandwich the carbon fiberbundle, and impregnating between the fibers of the carbon fiber bundlewith the resin. The evaluation results are shown in Table 9.

(2-5. Formation of Composite Panel (6 Ply))

The obtained unidirectional prepreg was cut into a size of a length (0°direction, direction parallel to the fiber) of 300 mm and a width (90°direction, direction perpendicular to the fiber) of 300 mm. Six prepregswere laminated with adjusting to the 0° direction and bagged, and then avacuum bag was formed under a curing condition in FIG. 6 using an ovento obtain a composite panel. The evaluation results are shown in Table9.

(2-6. Formation of Composite Panel (10 Ply))

The obtained unidirectional prepreg was cut into the size of the length(0° direction, direction parallel to the fiber) of 300 mm and the width(90° direction, direction perpendicular to the fiber) of 300 mm. Tenprepregs were laminated with adjusting to the 0° direction and bagged,and then a vacuum bag was formed under a curing condition in FIG. 5using an oven to obtain a composite panel.

(2-7. 0° Compression Test)

A test piece was made by adhering a tab formed from the same material asin the composite panel to the composite panel obtained above and thencutting it into a dimension of a length (0° direction) of 80 mm and awidth of 12.7 mm using a wet diamond cutter. A 0° compression test wasperformed in the obtained test piece using an all-round tester Instron5882 manufactured by Instron and analysis software Bluehill inaccordance with SACMA 1R-94 to calculate a 0° compression strength andan elastic modulus. The evaluation results are shown in Table 9.

(2-8. 0° Bending Test)

A test piece was made by cutting the composite panel (10 ply) obtainedabove into a dimension of a length (0° direction) of 127 mm and a width(90° direction) of 12.7 mm using the wet diamond cutter. A three-pointbending test was performed in the obtained test piece using an all-roundtester Instron 5565 manufactured by Instron and analysis softwareBluehill in accordance with ASTM D-790 (indenter R=5.0, L/D=40,crosshead speed: 5.26 to 5.22 mm/minute) to calculate a 0° bendingstrength and a 0° bending elastic modulus. The evaluation results areshown in Table 9.

(2-9. 90° Bending Test)

A test piece was made by cutting the composite panel (10 ply) obtainedabove into a dimension of a length (0° direction) of 25.4 mm and a width(90° direction) of 50 mm using the wet diamond cutter. A three-pointbending test was performed in the obtained test piece using theall-round tester Instron 5565 manufactured by Instron and analysissoftware Bluehill in accordance with ASTM D-790 (indenter R=3.2, L/D=16,crosshead speed: 0.838 to 0.902 mm/minute) to calculate a 90° bendingstrength and a 90° bending elastic modulus. The evaluation results areshown in Table 9.

(2-10. Interlayer Shearing Test)

A test piece was made from the composite panel (10 ply) obtained aboveusing the wet diamond cutter, and an interlayer shearing strength wasmeasured in accordance with ASTM D-2344. The evaluation results areshown in Table 9.

Example 31

The “PAN-based carbon fiber 2” having the fineness of 1.24 dtex wasused. Both the strength and the elastic modulus exhibited the highvalues in the 0° compression test. A strength retention rate of thestrength in the 10 ply composite panel relative to the strength in the 6ply composite panel (=10 ply panel strength/6 ply panel strength×100) inthe 0° compression test exhibited a high value (97.6%).

Example 32

The “PAN-based carbon fiber 3” having the fineness of 2.01 dtex wasused. Both the strength and the elastic modulus exhibited the highvalues in the 0° compression test. The strength retention rate of thestrength in the 10 ply composite panel relative to the strength in the 6ply composite panel in the 0° compression test exhibited a high value(98.7%).

Comparative Example 28

The “PAN-based carbon fiber 4” having the fineness of 1.21 dtex wasused. The strength of the 6 ply composite panel was lower than that inExample 31 in the 0° compression test, and this strength was not atusable level.

Comparative Example 29

The “PAN-based carbon fiber 5” having the fineness of 2.29 dtex wasused. The strength of the 6 ply composite panel was lower than that inExample 32 in the 0° compression test, and this strength was not atusable level.

Comparative Example 30

The “PAN-based carbon fiber 1” having the fineness of 0.75 dtex wasused. The strength of the 10 ply composite panel was lower than that inExample 32 in the 0° compression test, and this strength was not atusable level. The strength retention rate of the strength in the 10 plycomposite panel relative to the strength in the 6 ply composite panel inthe 0° compression test exhibited a low value (82.5%).

Comparative Example 31

This Comparative Example is an example in which the resin viscosity inComparative Example 30 was increased. The flow index was reduced to 1941Pa⁻¹, the strength of the 10 ply composite panel exhibited the lowvalue, the strength retention rate of the strength in the 10 plycomposite panel relative to the strength in the 6 ply composite panel inthe 0° compression test was slightly enhanced, but the strengthretention rate exhibited the low value (87.1%).

Comparative Example 32

This Comparative Example is an example in which the curing rate inComparative Example 30 was increased. The flow index was reduced to 2123Pa⁻¹, the strength of the 10 ply composite panel exhibited the lowvalue, the strength retention rate of the strength in the 10 plycomposite panel relative to the strength in the 6 ply composite panel inthe 0° compression test was slightly enhanced, but the strengthretention rate exhibited the low value (88.0%).

According to the present invention, it is possible to evenly treat theprecursor fiber bundle having the large single-fiber fineness and theexcellent productivity without reducing the productivity in theflame-proof treatment step, and further it is possible to obtain thecarbon fiber bundle with high quality containing few interlaced singlefibers in the fiber bundle and having the excellent spreadability.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Example 7 Example 8 Copolymer Copolymer A Copolymer C Copolymer ACopolymer F Copolymer A Copolymer G Copolymer G Copolymer G Monomer ANMol % 98.7 98.8 98.7 98.4 98.7 98.3 98.3 98.3 supply HEMA Mol % 1.3 —1.3 — 1.3 1.7 1.7 1.7 composition ratio HPMA Mol % — 1.2 — — — — — — HEAMol % — — — 1.6 — — — — AAm Mol % — — — — — — — — MAA Mol % — — — — — —— — IBMA Mol % — — — — — — — — Water/monomer Mass ratio 3.0 3.0 3.0 3.03.0 3 3 3 Copolymer AN Mol % 98.5 98.8 98.5 98.5 98.5 98.0 98.0 98.0composition HEMA Mol % 1.5 — 1.5 — 1.5 2.0 2.0 2.0 ratio HPMA Mol % —1.2 — — — — — — HEA Mol % — — — 1.5 — — — — AAm Mol % — — — — — — — —MAA Mol % — — — — — — — — IBMA Mol % — — — — — — — — Copolymer Specificviscosity ηap — 0.21 0.21 0.21 0.21 0.21 0.22 0.22 0.22 characteristicsMelting point under heat and humidity ° C. 170 172 170 170 170 168 168168 Film Water ° 82.3 62.8 62.3 62.5 62.3 58.0 58.0 58.0 contact angleOxidation μm 4.5 4.5 4.5 4.5 4.5 4.7 4.7 4.7 depth Precursor Singlefiber fineness dtex 2.0 2.0 2.5 2.5 3.0 2.5 2.5 2.5 fiber bundleFilament number Number 30000 30000 24000 24000 20000 24000 24000 24000Fiber density g/cm³ 1.180 1.180 1.180 1.180 1.180 1.180 1.180 1.180Spinning bath concentration wt % 60 60 60 60 60 60 50 50 Spinning bathtemperature ° C. 35 35 35 35 35 35 40 35 Roundness — 0.85 0.86 0.83 0.820.82 0.90 0.88 0.86 Cross-sectional shape — Horsebean HorsebeanHorsebean Horsebean Horsebean Horsebean Horsebean Horsebean Heat fluxtype Heat kJ/kg 185 183 177 187 163 168 168 170 differential quantity Jascanning Heat kJ/kg 740 780 732 825 715 722 720 725 calorimeter quantityJb Flame-proof Treatment time min 60 60 60 60 90 90 90 70 treatmentTemperature ° C. 250~290 250~290 250~290 250~290 250~290 240~270 240~270240~260 Extending rate % +2 +2 +2 +2 +2 +2 +2 +2 Flame-proofed fiberdensity g/cm³ 1.392 1.391 1.406 1.395 1.398 1.397 1.400 1.351 Carbonfiber Single fiber fineness dtex 1.07 1.06 1.36 1.34 1.61 1.34 1.35 1.25bundle Diameter Di μm 9.43 9.45 10.9 10.84 11.21 10.87 11.27 11.42Roundness — 0.84 0.85 0.82 0.81 0.83 0.88 0.85 0.83 Fiber spreading mm23.3 23.4 26 26.2 28.8 28.7 27.8 27.6 Strand strength MPa 4300 4250 39704100 3800 4240 4270 4350 Strand elastic modulus GPa 245 240 245 240 240250 245 239

TABLE 2 Example 9 Example 10 Example 11 Example 12 Example 13 Example 14Example 15 Copolymer Copolymer G Copolymer G Copolymer G Copolymer GCopolymer G Copolymer A Copolymer B Monomer supply composition ratio ANMol % 98.3 98.3 98.3 98.3 98.3 98.7 97.9 HEMA Mol % 1.7 1.7 1.7 1.7 1.71.3 2.1 HPMA Mol % — — — — — — — HEA Mol % — — — — — — — AAm Mol % — — —— — — — MAA Mol % — — — — — — — IBMA Mol % — — — — — — — Water/monomerMass ratio 3 3 3 3 3 3.0 3.0 Copolymer composition ratio AN Mol % 98.098.0 98.0 98.0 98.0 98.5 97.5 HEMA Mol % 2.0 2.0 2.0 2.0 2.0 1.5 2.5HPMA Mol % — — — — — — — HEA Mol % — — — — — — — AAm Mol % — — — — — — —MAA Mol % — — — — — — — IBMA Mol % — — — — — — — Copolymercharacteristics Specific viscosity ηap — 0.22 0.22 0.22 0.22 0.22 0.210.21 Melting point under heat and humidity ° C. 168 168 168 168 168 170165 Film Water ° 58.0 58.0 58.0 58.0 58.0 62.3 53 contact angleOxidation μm 4.7 4.7 4.7 4.7 4.7 4.5 4.9 depth Precursor fiber bundleSingle fiber fineness dtex 2.5 2.5 2.5 2.5 3.0 4.0 4.5 Filament numberNumber 24000 24000 24000 24000 20000 15000 15000 Fiber density g/cm³1.180 1.180 1.180 1.180 1.180 1.180 1.181 Spinning bath concentration wt% 45 45 45 30 45 40 45 Spinning bath temperature ° C. 45 35 25 35 35 3535 Roundness — 0.90 0.84 0.83 0.82 0.85 0.83 0.86 Cross-sectional shape— Horsebean Horsebean Horsebean Horsebean Horsebean Horsebean HorsebeanHeat flux type Heat kJ/kg 170 172 175 177 174 187 190 differentialquantity Ja scanning Heat kJ/kg 724 730 740 733 721 745 758 calorimeterquantity Jb Flame-proof treatment Treatment time min 70 70 90 90 70 9090 Temperature ° C. 240~260 240~260 240~270 240~270 240~260 250~290250~290 Extending rate % +2 +2 +2 +2 +2 +2 +2 Flame-proofed fiberdensity g/cm³ 1.355 1.352 1.402 1.405 1.354 1.360 1.360 Carbon fiberbundle Single fiber fineness dtex 1.26 1.25 1.35 1.36 1.50 2.03 2.28Diameter Di μm 11.54 11.88 12.31 12.38 12.27 12.35 15.62 Roundness —0.87 0.82 0.79 0.78 0.82 0.80 0.83 Fiber spreading mm 28.1 30.8 32.031.8 34.0 42.5 43.9 Strand strength MPa 4160 4150 4100 4100 4210 37003600 Strand elastic modulus GPa 241 249 251 255 242 235 232

TABLE 3 Compara- Compara- Compara- Comparative Comparative ComparativeComparative tive tive tive Example 1 Example 2 Example 3 Example 4Example 5 Example 6 Example 7 Copolymer Copolymer Copolymer Copolymer ECopolymer E Copoly- Copoly- Copoly- A D mer E mer H mer I Monomer AN Mol% 98.7 98.5 97.0 97.0 97.0 98.2 98.1 supply HEMA Mol % 1.3 — — — — — —composition AAm Mol % — — 2.6 2.6 2.6 — — ratio MAA Mol % — — 0.4 0.40.4 — — IA Mol % — — — — — 0.7 0.5 IBMA Mol % — 1.5 — — — 1.1 — MMA Mol% — — — — — — 1.4 Water/monomer Mass 3.0 3.0 3.0 3.0 3.0 3.0 3.0 ratioCopolymer AN Mol % 98.5 97.3 97.5 97.5 97.5 97.7 97.5 composition HEMAMol % 1.5 — — — — — — ratio AAm Mol % — — 2.0 2.0 2.0 — — MAA Mol % — —0.5 0.5 0.5 — 0.7 IA Mol % — — — — — 0.8 — IBMA Mol % — 2.7 — — — 1.5 —MMA Mol % — — — — — — 1.8 Copolymer Specific viscosity ηap — 0.21 0.210.21 0.21 0.21 0.22 0.20 characteristics Melting point under ° C. 170158 168 168 168 170 160 heat and humidity Film Water ° 62.3 74.4 58.158.1 58.1 76.2 71.1 contact angle Oxidation μm 4.5 6.4 3.0 3.0 3.0 4.53.2 depth Precursor Single fiber fineness dtex 1.5 2.0 2.0 2.5 4 2.5 3.0fiber Filament number Number 40000 30000 30000 24000 15000 24000 20000bundle Fiber density g/cm³ 1.180 1.176 1.180 1.18 1.18 1.18 1.18Spinning bath concentration wt % 67 60 60 45 45 45 45 Spinning bathtemperature ° C. 45 35 35 35 35 35 35 Roundness — 0.94 0.86 0.85 0.830.81 0.84 0.82 Cross-sectional shape — Round Horsebean HorsebeanHorsebean Horsebean Horsebean Horsebean Heat flux type Heat kJ/kg 185 52190 190 190 178 262 differential quantity Ja scanning Heat kJ/kg 743 3401150 1151 1151 473 512 calorimeter quantity Jb Flame-proof Treatmenttime Minute 30 60 60 60 100 100 70 treatment Temperature ° C. 250~290250~290 230~270 230~270 210~240 210~290 230~260 Density of g/cm³ 1.4191.392 1.405 1.396 1.350 1.352 1.361 flame-proofed fiber Carbon fiberSingle fiber fineness dtex 0.82 1.07 1.09 1.35 1.99 1.25 1.71 bundleDiameter Di μm 7.6 9.4 9.4 11.9 Cannot 11.7 12.3 Roundness — 0.95 0.850.81 0.82 be 0.82 0.81 Fiber spreading mm 20.0 23.3 23.4 30.8 sampled30.1 34.0 Strand strength MPa 1910 2450 3100 3700 2800 3200 Strandelastic modulus GPa 222 198 215 210 195 205

TABLE 4 Compara- Compara- Compara- tive tive tive ComparativeComparative Comparative Comparative Exam- Exam- Exam- Example 8 Example9 Example 10 Example 11 ple 12 ple 13 ple 14 Copolymer Copolymer JCopolymer Copolymer Copolymer Copoly- Copoly- Copoly- G B B mer G mer Gmer G Monomer AN Mol % 95.5 98.3 97.9 97.9 98.3 98.3 98.3 supply HEMAMol % — 1.7 2.1 2.1 1.7 1.7 1.7 composition AAm Mol % 2.5 — — — — — —ratio MAA Mol % — — — — — — — IA Mol % — — — — — — — IBMA Mol % 2 — — —— — — MMA Mol % — — — — — — — Water/monomer Mass 3.0 3.0 3.0 3.0 3.0 3.03.0 ratio Copolymer AN Mol % 95.5 98.0 97.5 97.5 98.0 98.0 98.0composition HEMA Mol % — 2.0 2.5 2.5 2.0 2.0 2.0 ratio AAm Mol % 2.0 — —— — — — MAA Mol % — — — — — — — IA Mol % — IBMA Mol % 2.5 — — — — — —MMA Mol % — — — Copolymer Specific viscosity ηap — 0.21 0.22 0.21 0.210.22 0.22 0.22 characteristics Melting point under ° C. 155 168 165 165168 168 168 heat and humidity Film Water ° 56.2 58.0 53.0 53.0 58.0 58.058.0 contact angle Oxidation μm 6.3 4.7 4.9 4.9 4.7 4.7 4.7 depthPrecursor Single fiber fineness dtex 2.5 1.0 2.0 2.5 2.5 3.0 4.0 fiberFilament number Number 24000 60000 30000 24000 24000 20000 15000 bundleFiber density g/cm³ 1.18 1.180 1.180 1.181 1.180 1.180 1.180 Spinningbath concentration wt % 45 45 67 67 67 67 67 Spinning bath temperature °C. 35 35 45 45 35 45 45 Roundness — 0.85 0.85 0.97 0.98 0.95 0.98 0.98Cross-sectional shape — Horsebean Horsebean Round Round Round RoundRound Heat flux type Heat kJ/kg 82 170 183 186 163 159 157 differentialquantity Ja scanning Heat kJ/kg 1098 718 740 750 710 698 694 calorimeterquantity Jb Flame-proof Treatment time Minute 90 70 60 60 70 70 70treatment Temperature ° C. 240~270 240~260 250~290 250~290 240~260240~260 240~260 Density of g/cm³ 1.402 1.362 1.352 1.398 1.352 1.3511.354 flame-proofed fiber Carbon fiber Single fiber fineness dtex 1.350.51 1.00 1.35 1.25 1.50 2.01 bundle Diameter Di μm 11.9 7.1 7.9 9.610.2 10.4 11.6 Roundness — 0.83 0.84 0.96 0.95 0.91 0.96 0.95 Fiberspreading mm 30.5 20.9 21.2 25.2 25.5 28.3 33.9 Strand strength MPa 35104350 3540 3150 2800 2400 2250 Strand elastic modulus GPa 210 262 210 210226 190 175

TABLE 5 Single fiber fineness Precursor Solidification Solidification ofprecursor fiber Acryl-based Acryl-based copolymer bath concentrationbath temperature fiber bundle bundle copolymer composition [Mass %] [°C.] [dtex] A A AN/HEMA 45 35 2.5 B (HEMA2 Mol %) 67 45 2.5 C 67 35 2.5 D45 45 2.5 E 45 25 2.5 F 60 35 3 G 45 35 3 H 45 35 1.5 I 40 35 4 J BAN/HEMA 60 35 2 (HEMA1.5 Mol %) K D AN/AAm/MAA 60 35 2.5 L 60 35 1.5

TABLE 6 Single Calorific 1H-NMR Density of Strand Precursor fiber valueper Half-value Flame-proof flame-proofed Strand elastic fiber finenessunit mass width time fiber strength modulus bundyle [dtex] [kJ/kg] [kHz][Minute] [g/cm3] [MPa] [GPa] Example 16 A 2.5 3400 12.5 70 1.352 4390251 Example 17 90 1.400 4280 260 Example 18 B 2.5 3220 12.6 70 1.3454280 232 Example 19 90 1.395 4120 241 Example 20 C 2.5 3250 12.5 701.352 4210 241 Example 21 90 1.392 4150 247 Example 22 D 2.5 3360 12.570 1.355 4240 246 Example 23 90 1.398 4350 248 Example 24 E 2.5 346012.5 70 1.352 4320 260 Example 25 90 1.402 4180 256 Comparative Example15 F 3 2970 12.5 70 1.351 4160 223 Comparative Example 16 90 1.388 1460221 Example 26 G 3 3270 12.5 70 1.349 4300 247 Example 27 90 1.400 4140245 Comparative Example 17 H 1.5 3930 12.5 70 1.365 4700 261 ComparativeExample 18 90 1.414 4410 257 Comparative Example 19 I 4 2950 12.5 701.348 3490 222 Example 28 J 2 3600 13.2 60 1.353 4780 257 ComparativeExample 20 K 2.5 3160 14.9 60 1.354 3800 216 Comparative Example 21 L1.5 3730 15 60 1.358 4900 260

TABLE 7 Comparative Comparative Comparative Comparative ComparativeComparative Example 29 Example 30 Example 22 Example 23 Example 24Example 25 Example 26 Example 27 AN (Mol %) 98.5 98 98.5 96 96 96 96 96HEMA (Mol %) 1.5 2 1.5 — — — — — AAm (Mol %) — — — 3 3 3 3 3 MAA (Mol %)— — — 1 1 1 1 1 Hole number of spinning nozzle 24000 24000 12000 2400060000 12000 3000 24000 Hole diameter of spinning nozzle 60 60 60 60 4575 75 75 (μm) Discharge magnification (times) 0.32 0.32 0.18 0.32 0.360.59 0.61 0.51 Stretching magnification (times) 5.4 5.4 5.4 5.4 5.3 4.85.9 5.4 upon washing with water Dry heat stretching magnification 1.341.34 1.34 1.3 1.7 1.34 (times) Steam stretching magnification 2.7 2.1(times) Total stretching magnification 7.4 7.4 7.4 7.3 9 12.7 12.5 7.4(times) Flame-proof time (minutes) 70 70 300 300 70 60 60 60 Carbonfiber fineness (dtex) 1.3 1.3 2.4 1.4 0.53 0.67 0.69 Roundness 0.75 0.750.73 0.75 0.63 0.95 0.81 Depth of wrinkle (nm) 49.8 50 41.9 45 32.2 94123 Passing through burning step ◯ ◯ ◯ ◯ ◯ ◯ ◯ X Strand strength (Mpa)4300 4200 3200 3800 4900 5000 4400 Strand elastic modulus (Gpa) 233 232231 240 255 244 244 Elevation height (mm) 126 125 163 125 90 82 80 Resinimpregnation time (minutes) 9 9 9 9 15 15 12 Impregnation with resin ◯ ◯◯ ◯ X X X

TABLE 8 Comparative Comparative Comparative Comparative ComparativeExample 31 Example 32 Example 28 Example 29 Example 30 Example 31Example 32 resin jER828 55 55 55 55 55 40 55 AER4152 45 45 45 45 45 6045 Thermoplastic resin Vinylec E 2 2 2 2 2 2 2 Curing aid DCMU 1.33 1.331.33 1.33 1.33 1.33 — PDMU — — — — — — 4 Curing agent DICY 5.33 5.335.33 5.33 5.33 5.33 5.33

TABLE 9 Comparative Comparative Comparative Comparative Comparative UnitExample 31 Example 32 Example 28 Example 29 Example 30 Example 31Example 32 CF Weight per unit mg/m 2970 2768 2899 2752 3750 3750 3750Density g/cm³ 1.804 1.779 1.809 1.776 1.81 1.81 1.81 Strength MPa 42263489 3989 3283 4116 4116 4116 Elastic modulus GPa 229 246 227 232 235235 235 Fiber number Number/tow 24000 12000 24000 12000 50000 5000050000 Single fiber dtex 1.24 2.01 1.21 2.29 0.75 0.75 0.75 finenessRoundness 0.75 0.83 0.95 0.95 0.70 0.70 0.70 Resin Flow rate 1/Pa 54205420 5420 5420 5420 1941 2123 Prepreg FAW g/m² 213 211 208 202 210 203205 RC % 33.9 34.0 32.2 32.8 33.9 33.1 34.3 0° Laminated number ply 6 106 10 6 6 6 10 6 10 6 10 Compression Strength MPa 1424 1390 1420 14021306 1335 1541 1271 1549 1349 1521 1339 [Note 1] Elastic modulus GPa 118119 119 113 111 112 118 117 119 117 117 115 Strength % 97.6 98.7 82.587.1 88.0 retention rate [Note 2] [Note 1] Vf56% conversion [Note 2]Strength retention rate = 10 ply strength/6 ply strength × 100

EXPLANATION OF REFERENCE NUMERALS

5: Carbon fiber bundle

6: Impregnation height

7: Clip

8: Ruler

9: Formamide

10: Angle

11: Formimg die

12: Carbon fiber fabric

13: Spiral tube

14: Medium

15: Peel ply

16: Bag film

17: Sealing material

18: Suction spout of vacuum pump

19: Valve

20: Discharge spout of resin

INDUSTRIAL APPLICABILITY

The carbon fiber bundle of the present invention can be used in manyfields including materials for aviation and aerospace such as airplanesand rockets, materials for sports such as tennis rackets, golf shaftsand fishing rods, materials for transporting machines such as ships andautomobiles, materials for electronic parts such as housing parts ofmobile phones and personal computers, and materials for electrodes offuel cells.

1. A carbon fiber bundle, wherein an average single-fiber fineness isfrom 1.0 to 2.4 dtex and a roundness is from 0.7 to 0.9 in a shape of across-section perpendicular to a fiber axis of a single fiber, whereinthe carbon fiber bundle has a plurality of groove-shaped concavo-convexstructures extending in a lengthwise direction of the single fiber on asurface of the single fiber, and wherein a difference in height betweena highest part and a lowest part of a circumference length of 2 μm ofthe single fiber is 50 nm or more.
 2. The carbon fiber bundle accordingto claim 1, wherein a diameter Di of the cross-section perpendicular tothe fiber axis of the single fiber is from 8 to 20 μm.
 3. The carbonfiber bundle according to claim 1, wherein the difference in height is80 nm or less.
 4. The carbon fiber bundle according to claim 1, whereina strand tensile strength is 4000 MPa or more.
 5. The carbon fiberbundle according to claim 1, wherein a strand tensile elastic modulus is200 GPa or more.
 6. The carbon fiber bundle according to claim 1,wherein a total fineness is from 30000 to 90000 dtex.